The Marvel Cinematic Universe is easily the most successful and well-put-together example of a shared film universe in the history of film. But, it was also the first series of films to interconnect in such a way, and thus it's had its fair share of slip-ups. Sometimes it's a fun easter egg that has to be retconned, sometimes it's a thread left completely hanging. Whatever it is, there are quite a few mysteries within the Marvel Cinematic Universe. We've investigated a few of them before, but we didn't cover everything, and since that first list, there have been quite a few more Marvel films, including Infinity War, which has left us with plenty of new questions.

So, what are some of the biggest mysteries in the MCU? With all the meticulous planning and communication, one might think that Marvel has gotten this down by now. Then again, no one is perfect, and the MCU has left quite a few unresolved issues, unsolved mysterious, hanging threads and overall continuity blunders that have left us with a few burning questions about the Marvel films. You can of course take all of these with a grain of salt, since some of these mysteries may be seeds planted for later storylines, while others are small mistakes that might not lead anywhere. Whatever the reason, here is CBR's list of 15 Mysteries That Still Remain In The MCU.

15 SPIDER-MAN HOMECOMING TIMELINE

Spider-Man being in the MCU is a bit strange when you think about it. Spider-Man: Homecoming was a Sony film, but it took place in the MCU and the film made a point to show just how connected it was to the rest of the films. There was the fact that Iron Man was a major supporting character, all the easter eggs in the cargo ship and the fact that the film's villain was essentially created from the aftermath of the Battle of New York. And speaking of the climax of The Avengers, the film, it is also responsible for one of Spider-Man: Homecoming's biggest mysteries, its timeline.

Well, perhaps "mystery" isn't exactly the right word, since it's more of a slip-up than anything.

Regardless, we are aware, through various sources including an official MCU timeline infographic released by Marvel, that that The Avengers and Captain: America: Civil War take place in the years they were released, 2012 and 2016 respectively. Yet, in the opening for Homecoming, we find Adrian Toomes and his crew cleaning up after the Battle of New York, followed by an "8 years later" title card. But, that would mean that Homecoming took place in 2020, despite the fact that it's supposed to take place a short time after the events of Civil War. Marvel has admitted this was a mistake, but it still leaves the question of when Homecoming actually took place.

14 WHAT HAPPENED TO BETA RAY BILL?

Fans of the MCU love searching for easter eggs, especially references hidden in background details. These are some of the most interesting parts of the MCU, since they imply characters and storylines from the comics have to exist within the larger world of the films. One example of this is Beta Ray Bill, who has been featured in two major easter eggs in the MCU. The first came in Guardians of the Galaxy as one of The Collector's possessions. However, there wasn't anything explicitly stating that this was Beta Ray Bill, though in the Guardians of the Galaxy: Mission Breakout ride, we can see Beta Ray Bill's "Asgardian Hammer" (since Stormbreaker was introduced in Infinity War).

Of course, the Mission Breakout ride might not exactly be an "official" entry in the MCU, but there is still a larger easter egg we've yet to cover, the one featured in Thor Ragnarok. On the tower depicting the champions of Sakaar, Beta Ray Bill's head can be seen among them. This leads us to wonder what exactly happened to the character, and we might have an answer. Perhaps Beta Ray Bill angered the Grandmaster during his time as a champion, and in turn the Grandmaster gave him to his brother, The Collector, to be kept prisoner as punishment for whatever he did. Perhaps we will see more of Beta Ray Bill in later movies and finally get some definitive answers.

13 BIFROST MYSTERIES

The Bifrost from Thor

The Bifrost plays a big role in the first and third Thor films, and as such there is a lot of weight to its power and those who have access to it. The Bifrost is the bridge that allows transport between the nine realms, which might make it seem like it is of the utmost importance and is capable of power that cannot be replicated, but this isn't exactly true. In fact, the Bifrost basically lost all importance the second Thor showed up in The Avengers. Heck, we could even go back farther and say that it lost importance the second Odin used his spear to open the bifrost, since it proved that Heimdall's sword wasn't the only key.

Why is the Bifrost depicted as being the only way of transporting to the other realms? We've seen quite a few examples that prove otherwise.

And yet, Hela sought out Heimdall's sword as the key, despite the fact that she knew her father's Spear would have the same effect. Furthermore, why is the Bifrost depicted as being the only way of transporting to the other realms? We've seen quite a few examples that prove otherwise, one being the Tesseract, which Loki and Thor used to get home at the end of The Avengers and when Thor showed up to Earth using Stormbreaker in Infinity War. This leaves us with a mystery, both in the sense of what the Bifrost is and what capabilities of Stormbreaker and the Tesseract allow them to access the Bifrost's teleportation.

12 THE ROSS FAMILY

Thunderbolt Ross from Incredible Hulk

Perhaps one of the biggest disconnects in the Marvel Cinematic Universe comes from The Hulk. Specifically, we are talking about the fact that the events of The Incredible Hulk seem to be relatively ignored by the rest of the MCU films. Harlem seems unaffected by his battle with Abomination, Abomination and Leader have been completely abandoned as villains and the Bruce Banner we see from Avengers onward seems to be a completely different person than the Edward Norton version. Of course, people can change and Bruce Banner has every reason to start anew, but the other disconnects make the film seem like an outlier amongst the rest of the MCU.

Perhaps the biggest disconnect between The Incredible Hulk and the rest of the MCU films has to do with Lt. Ross and his daughter Betty. Despite being a love interest for Bruce Banner, Betty Ross completely disappeared from the MCU after the events of The Incredible Hulk. This is somewhat problematic since Bruce is no longer an exile by the time of The Avengers, and even if the two don't get back together, there should at least be a scene depicting them meeting since their last encounter, especially since The Hulk is a bit of a superhero celebrity at this point. Speaking of that, why don't we see any interaction between Bruce Banner and Lt. Ross? Does he approve of The Hulk being seen widely as a hero?

11 DOCTOR STRANGE TIMELINE

Doctor Strange Time Stone Eye of Agamotto 2

There is a bit of irony in the fact that time played a major role in the climax of Doctor Strange, since the timeline of the film is rather hard to figure out. First of all, it is hard to figure out when exactly in the MCU the film takes place, which doesn't exactly matter that much, since it doesn't interfere with any of the other movies' timelines, but it is still a mystery that some have tried to figure out. The fact that Strange is part of Infinity War, as well as his cameo in Thor: Ragnarok, makes things a bit more difficult, since it means that Strange was a fully-trained master sorcerer by this point, something that also creates its own timeline issues.

The amount of time it took him to master the mystic arts isn't perfectly clear.

This is to say that, even with how smart Stephen Strange is, the amount of time it took him to master the mystic arts isn't perfectly clear. This is another one that fans have tried to figured out with mixed results, since, as great as Strange's eidetic memory is, it seems next to impossible for him to master an ancient and widely unknown art in under a year, especially since his first attempts at magic were utter failures. Furthermore, how much farther into the future did the post credits scene of the film take place? Had he been the guardian of the New York Sanctum for some time? It is all a bit vague.

10 DID THE SOKOVIA ACCORDS PASS?

The Sokovia accords in the MCU

This is perhaps one of the biggest mysteries of the entire MCU, one that is completely glossed over without any real consequences being shown. Perhaps it is because bigger matters have been thrust into the spotlight since they were proposed, but we never learned if the Sokovia Accords actually passed. Like we said, there were bigger matters at hand during and after the events of Captain America: Civil War, but that might not be the only reason we haven't heard anything about the Sokovia accords during the film. The Accords might not have passed thanks to lack of support.

See, in the climactic battle between Team Cap and Team Iron Man, there was a very clear, very divided line between those who supported the Accords and those who didn't. And yet, as the battle and the film went on, we found that, really, the only people who supported the accords were Iron Man, War Machine, Vision and maybe Spider-Man. We say maybe since Spider-Man might have been too young to sign them, but at the end of the day there's a lot of evidence that says the Accords didn't actually pass. For one thing, Iron Man encouraged Peter Parker to continue doing vigilante work despite supporting the Accords, which would have made that illegal. It would also make his own actions in Homecoming and Infinity War illegal, which leads us to believe the Accords never passed.

9 THE STATE OF STARK INDUSTRIES

After the death of his father, Tony Stark took over as acting CEO of Stark Industries, and in the first Iron Man film, he changed the direction of the company after his learning what became of its weapons. Shutting down its weapons manufacturing branch, Stark industries focused on... something? While we know that the company is a massive tech conglomerate that produces much more than weapons, it has never been clear what its current direction is.

Tony Stark wanted to focus on the arc reactor technology and bringing it to the world... but did that ever happen?

After Tony appointed Pepper Potts as the company's new CEO, news and updates on the state of Stark Industries have been rather sparse. We can assume the company does well enough to provide Tony and the Avengers with all the tech they need. But, from what we can tell, nothing ever really came of advancing the arc reactor tech beyond powering Iron Man suits. We also know that Stark Industries is helping college students with their projects, as seen in Civil War, and thanks to Agents of S.H.I.E.L.D., we know that they are also providing private security, which seems to be a bit of a veer from their technological focus. All these questions make us wonder what exactly the company is up to these days and what their focus is.

8 IRON PATRIOT OR WAR MACHINE?

It's not exactly clear where Rhodey's alliance lies. He is, of course, a close friend and ally of Tony Stark, but he is also a Military officer of the United States Air Force (well, pending his court-martial after Infinity War). Because of both of these alliances, Rhodey has been shown to both fulfill his duty as a soldier and back up his friend in a fight. But, this dichotomy raises a few important questions that make for one of the MCU's more interesting mysteries. The first major question is, why did Rhodey go back to the War Machine look and moniker instead of sticking with Iron Patriot?

There doesn't really seem to be a solid answer to this, and trying to speculate one only raises further questions. Rhodey has gone from being an ally of Tony's, to someone who was forced to stop him using a past suit of armor, to an agent of the Government armed with a heavily armed suit, to the Iron Patriot (still a government agent), then back to War Machine in Age of Ultron, then to a proponent of the Sokovia Accords as the liaison between the Avengers and the United Nations and in Infinity War, it seems like he has turned his back on the government once more. Perhaps it is because of his ever-changing alliances that Rhodey thought it best to go back to his War Machine moniker? Who's to say.

7 IS THERE A WORKING SUPER SOLDIER SERUM?

Captain America The First Avenger Chris Evans

Captain America's powers are kind of tame when compared to the likes of Thor or the Hulk, but the peak (and partially beyond) human strength, speed, durability, etc., that was given to him by the super soldier serum are all still rather impressive. So impressive, in fact, that the formula for Abraham Erskine's serum was heavily sought-after by a number of different people and organizations. But, the only complete formula was the one that was used on Steve Rogers... or is it?

See, one of the biggest mysteries in the MCU is wether or not the super soldier serum has been recreated successfully or not.

For one thing, we know that Bucky was given a form of the serum, and there were a few other Winter Soldiers who were given a version as well, one that was created by Howard Stark, which got him killed. Furthermore, Captain America's DNA contains the last known remnants of Abraham Erskine's serum, so what was stopping S.H.I.E.L.D. from taking his blood and making more super soldiers? We know they were willing to go to extremes in The Winter Soldier, and the fact that HYDRA was running them from the inside makes us question why they didn't secure Cap's blood for an army of super soldiers. We also know that the serum given to Emil Blonsky before being mutated into Abomination was as successful as Cap's serum, so is there or isn't there a working super soldier serum out there?

6 LOKI'S STAFF

The MCU spent years laying the groundwork for Infinity War, and those efforts succeeded beyond even the wildest imagination. However, there were a few stumbles along the way. The way infinity stones were introduced and teased  were clearly not 100% planned out from the start. What's our proof of this? Well, there's the fact that the Infinity Gauntlet was seen as a background easter egg in Odin's vault before major Infinity War plans had been made, which is why it was revealed to be a fake in Thor: Ragnarok. There is also the fact that some of the Infinity Stones were not explicitly said to be stones, and were sort of retconned into being items like the tesseract and Loki's staff.

In fact, Loki's staff, introduced in The Avengers, houses some of the biggest mysteries of the MCU, specifically in regards to its power. Perhaps it was originally meant to be the soul stone, since it gave Loki the power to bring others under his command, specifically by tapping their hearts with the tip of the blade, something that failed to work on Tony Stark due to his arc reactor implant. And yet, it turned out to be the mind stone, which raises the question of why Thanos would entrust an infinity stone he already had to someone who is known for mischief and trickery. Plus, why does the "mind"stone allow Vision to shoot beams through his head?

5 WHAT'S UP WITH THE MIND STONE?

What exactly was the mind stone encased in when it was turned into Loki's staff? What was it about the blue shell that allowed the mind stone to do all that it did? As we just mentioned, it gave Loki the power to bend the hearts of mortals to his whim, but that wasn't all. The mind stone served as the final piece in creating The Vision, essentially giving him life and a long list of powers. The mind stone, while in Loki's staff, also had the ability to give the Maximoff twins superhuman abilities, which raises one of the biggest questions/mysteries in the MCU, what exactly are the mind stone's powers?

We were never actually shown what the mind stone could do.

Think about it, we've seen what every other infinity stone is capable of; the power stone causes destruction and seems to enhance strength by imbuing others with great energy, the time stone allows for mastery of time, the space stone allows for control over matter and teleportation, the soul stone is a bit less obvious, but we know it plays a key in Thanos' plan to kill off half the universe's population. So, what about the mind stone? We know what it can do, but what is its specific purpose? We were never actually shown what the mind stone could do, which is perhaps why it was chosen as the last stone to be capture in Infinity War, since Thanos just needed it to complete the power of the Infinity Gauntlet. So, the mystery is still unsolved, what are the mind stone's exact powers?

4 ODIN'S UNDERUTILIZED TREASURES

There are two main locations in the MCU that provide a ton of opportunities for awesome easter eggs: the Collector's collection and Odin's vault on Asgard. The latter is as full of mysteries as it is treasures and easter eggs, since some of the items within it are questionable at times. There is the aforementioned Infinity Gauntlet, which, even though it's a fake, still raises the question of why Odin would have it. There is the theory that Odin made a fake to keep the peace, so that Asgardians would believe the nine realms are safe if the gauntlet was in the hands of Odin. But, at the same time, we know that the dwarves of Nidavellir made Thanos' gauntlet very recently, so there was never a gauntlet to begin with, save for the fake.

This isn't the only mystery of Odin's vault, since its contents beg the question of why no on used any of the ultra-powerful treasures within it. Of course, no one can now that Asgard was destroyed, but prior to the events of Thor: Ragnarok, why didn't Thor utilize any of the great and powerful weapons within his father's treasure room? Couldn't The Avengers have used some of them in the fight with the Chitauri? What about the fight against Ultron? Seems like a bit of a waste.

3 THE STATUS OF THE TEN RINGS

In our previous list looking at the mysteries of the Marvel Cinematic Universe, we wondered who the real Mandarin was, since the one in Iron Man 3 turned out to be nothing more than an actor pretending to be a terrorist. However, in a special Marvel one-shot titled All Hail the King, it was revealed that the true Mandarin was pissed that Trevor Slattery took his name and legacy, or rather, that Aldrich Killian had used the mythology of the Mandarin as a basis for Slattery's character. But The Mandarin, the real one that is, is only one piece of the puzzle -- the puzzle of the Ten Rings.

The operations, motives, influence and overall presence of The Ten Rings (a reference to classic Iron Man villain, The Mandarin) remain an MCU mystery!

The Ten Rings was first seen in the first Iron Man film during Tony Stark's ransom video. Behind Stark and his captors was a flag featuring the insignia of the Ten Rings. So, who are the Ten Rings? They are a terrorist organization shrouded in mystery, and they are easily one of the least explored aspects of the MCU. There have been a few agents of the Ten Rings, but aside from the first Iron Man film and the All Hail the King short, there has been little revealed about the Ten Rings, making their operations, motives, influence and overall presence in the MCU a mystery, one that might be explored farther down the line.

2 TONY STARK'S CHARGES

While we're on the subject of the first Iron Man film, let's talk about the thing that set it apart from most of the other MCU films, the fact that Tony Stark killed. He's not the only MCU superhero to have done so but his actions in the first Iron Man film make us wonder why Tony's crimes haven't been brought to light. He might have changed a lot since his first time donning the suit, since he was out to fix his mistakes and more or less get revenge on the people who used his weapons for terrorism, but it doesn't change the fact that Iron Man killed several people.

Tony might be able to get away with killing Obadiah Stane since it was out of self defense, but what about his actions in Afghanistan? He acted as a foreign vigilante, killing in a country that he is not a citizen of without facing any consequences later on. And what about when he faced Ivan Vanko? He and Rhodey killed the villain, who might have otherwise been stopped without deadly force. Perhaps the fact that Tony "got away" with these killings, justified or not, is part of the reason that he was in support of the Sokovia Accords, since it means his actions from then on would be sanctioned/controlled by the United Nations.

1 DOCTOR STRANGE'S GLOVES

This last entry is a bit of a fun one, so don't take it too seriously. There is a lot to love about the costume that Doctor Strange wore in the film -- it was an awesome translation from comic book page to screen and had a fantastic design, especially with how the elements tied into his abilities and training. However, it was missing two key elements from the comics costume, Doctor Strange's gloves and insignia. Luckily, we got an updated version of the costume that was featured in both the post-credits scene of Doctor Strange and in Strange's cameo in Thor: Ragnarok.

And yet, this costume randomly disappeared after Thor: Ragnarok.

This costume featured the character's signature gloves and the top of the costume had a pattern that looked similar to the insignia featured in the comics. It wasn't a bad look, the glove were a bit goofy sure, but the whole ensemble actually came together really well with these elements from the comics. And yet, this costume randomly disappeared after Thor: Ragnarok. Despite the fact that it was an upgrade of some sort, Doctor Strange's gloves and new suit were not featured in Infinity War, with no specific reason given as to why. Maybe it's just an alternate outfit that was in the wash, but who can say for sure?