Endgame Theory: Infinity Gauntlet Has Been Used Before
A single line of dialogue in Avengers: Endgame may hint that the Infinity Gauntlet has been used to recreate the universe before. The history of the Infinity Stones is still something of a mystery in the MCU, but they certainly appear to have been responsible for a lot of chaos and misfortune. The Celestials used the Power Stone to judge entire worlds and civilizations, the Dark Elves sought to undo the Big Bang using the Aether, and the Soul Stone seems to have lured countless people to commit heinous sacrifices on Vormir.
In modern times, the Infinity Stones slipped into legend, so much so that most people believed they were just a myth. Then they began to resurface, one at a time, and many of them were mysteriously connected with Earth. It appears as though something spectacular happened with the Infinity Stones, and that a group of beings linked to Earth in different ways decided to hide them away. Odin took possession of the Space Stone; the Masters of the Mystic Arts kept the Time Stone, and knew the location of the Power Stone too; and the Reality Stone was hidden in a dimension that was linked to Earth and the Nine Realms.
It's possible next year's Eternals movie will reveal the ancient history of the Infinity Stones, given it's partly set millions of years in the past and features the Celestials. But for now, one intriguing possibility is that the Infinity Stones have been gathered together before, perhaps even with the creation of an Infinity Gauntlet, and that they've been used to rewrite the universe.
To date, there has only been one account of the history of the Infinity Stones, and it was given by the Collector in Guardians of the Galaxy. There, he implied that the Infinite Stones predate the creation of the universe itself - although he believed they originally took a different form. "Before creation itself," the Collector explained, "there were six singularities. Then the universe exploded into existence, and the remnants of these systems were forged into concentrated ingots... Infinity Stones."
The Collector's account matches perfectly with the events of Thor: The Dark World. The Dark Elves seem to have ruled the last universe, one that appears to have been going through the last stages of heat death, and consisted of an infinite darkness. They somehow survived - perhaps through the use of the Aether, the Reality Stone - and they entered the new universe of brightness and beauty. They settled on the realm of Svartalfheim, and forever sought to wind the clock back and restore their dark universe.
All this makes one scene in Avengers: Endgame stand out as quite remarkable. Thanos had originally intended to erase half the life in the universe with a snap of his fingers, but his insane plans escalated when he correctly realized the survivors would never stop fighting to put matters right. He suggested that, instead of simply wiping out half of all living creatures, he would recreate the universe itself. "With the stones you've collected for me," Thanos declared, "[I will] create a new one. Teeming with life, but knows not what it has lost but only what it has been given. A grateful universe." Crucially, Thanos intended to use the Infinity Stones to ensure this new universe was unaware of its own past. Given the near-unlimited power of the Infinity Stones, he could quite easily have done this. And, disturbingly, it's possible it has been done before; that the Infinity Stones have been used in some cosmos-altering way, but that they were then used to erase all memory of it.
Surprisingly, there is comic book precedent for this idea. In his Ultimates 2 series, writer Al Ewing revealed that the Marvel Comics universe has had multiple previous incarnations, each killed and then recreated by a cosmic event. Of course, in the comics each iteration is represented by an avatar of existence itself. The first of these, the First Firmament, created the Celestials. The Celestials desired to imitate their creator and fashion life in their own image, but this led to a brutal conflict as they warred over just what that life should look like. The end of that civil war saw one group of Celestials detonate unimaginable weapons, destroying and then recreating the universe, unwittingly creating the Multiverse.
The current Marvel Comics universe is in its eighth incarnation, referred to as the Eighth Cosmos, and it was recreated from the ashes of what had gone before in Jonathan Hickman's Secret Wars event. This latest iteration is still in its early days, with aspects of the Multiverse continuing to form, and it's known for the creation of a rare mineral called ISO-8. Significantly, the majority of all living beings have absolutely no awareness that they were destroyed and recreated; there are hints that catch the attention of those with cosmic awareness, tiny inconsistencies that fascinate scientists and sorcerers alike. But, in general, most of the inhabitants of the Eighth Cosmos have no idea the universe was reborn.
If the cosmos has been reborn in the comics, there's no reason to assume the same couldn't be true in the MCU as well. As evidence, attentive viewers would need to look for tiny inconsistencies, the kind of things that only cosmic beings in the MCU would be aware of. And there is indeed one such inconsistency; the history of the Infinity Gauntlet itself. Thanos believed that he was the first person to come up with the idea of using multiple Infinity Stones in tandem, and the Dwarves of Nidavellir believed they had fashioned the first Infinity Gauntlet, as shown in Avengers: Infinity War.
But this blatantly contradicts the Thor trilogy, where Odin had an Infinity Gauntlet stored within his Vault on Asgard. Hela dismissed this as a fake in Thor: Ragnarok, but that isn't a particularly satisfying solution to the problem; because what is a fake of? The Infinity Gauntlet must predate Thanos in some form, and on some level Odin and possibly even Hela must know it. If so, it was used on such a scale as to leave a race-memory running through the Dwarves of Nidavellir, so they unwittingly repeated its construction when Thanos came to them.
Right now, this is only an intriguing theory, and an exciting possibility. But post Avengers: Endgame, the MCU is about to jump back millions of years, to the dawning days of life itself, and Eternals may well finally reveal what happened to the Celestials. Given the civil war between that race tore the universe apart in the comics, it's safe to assume they are intimately connected to any similar cosmic rebirth plot in the MCU as well. If there is any truth to this theory, it will likely be revealed in 2020.