Spiral Nemesis

The Spiral Nemesis (スパイラル・ネメシス, Supairaru Nemeshisu?) is a theoretical apocalyptic event involving the overuse of Spiral Power, of which the Anti-Spiral fought to prevent.

Description
As the Anti-Spiral itself explains it, the Spiral Nemesis's catalyst is Spiral Power's ability to defy the laws of physics, such as creating matter from nothing, from Giga Drill Breaks to the transformation of the higher levels of Gunmen. If enough Spiral Power was to be generated, all Spiral lifeforms in the universe would eventually start evolving into full-sized galaxies. The sudden imbalance of mass in the universe would lead to these mega-galaxies devouring each other, forming a black hole which would consume all of existence. The Anti-Spiral's actions throughout the series are intended to destroy all Spiral Power in the universe, thereby keeping the Spiral Nemesis from ever happening.

When Team Dai-Gurren finishes off the Anti-Spiral, he accepts his fate and asks Simon to do whatever he can to prevent the Spiral Nemesis, to which Simon obliges.

In The Lights in the Sky are Stars, it is shown that the Spiral Nemesis is more than just a theory. When Super Tengen Toppa Gurren Lagann and Super Granzeboma's Giga Drill Breaks collide, the entire Anti-Spiral space (a multiverse consisting of thousands of universes) begins to collapse into itself. Of course, this was on an order of magnitude far greater than the Anti-Spiral thought the Spiral races were capable of. This shows that the Spiral Nemesis, while still theoretically possible, requires a great deal more mass and Spiral Power than even the Anti-Spiral had initially surmised.

Debate (WARNING: NON-CANON SPECULATION BELOW)
The nature and likelihood of the Spiral Nemesis is one of the biggest lingering questions in Tengen Toppa Gurren Lagann. Team Dai-Gurren appears to believe the Anti-Spiral's theory, resolving to either prevent or somehow defeat the event if it ever becomes a problem, but whether or not the Spiral Nemesis is a real danger is never definitively answered, and is left up to interpretation.

Among the fandom, several theories about the Spiral Nemesis have been proposed: Each of these theories - as well as others - have their believers and detractors, but the true nature of the Spiral Nemesis is and likely always will be a mystery.
 * The Spiral Nemesis is indeed a real threat, but if Spiral Power is not overused, it can be prevented. This seems to be the most common theory, and the one most people think the show implies.
 * The Spiral Nemesis is an inevitable consequence of evolution; when the universe's life forms grow too advanced, their combined Spiral Power will destroy everything. Limiting Spiral Power certainly delays it, but there is no way to totally circumvent the Spiral Nemesis.
 * The Spiral Nemesis is part of the universal life cycle; after everything has been devoured by the all-encompassing black hole, a new Big Bang will occur, beginning creation anew.
 * The Spiral Nemesis is a possible outcome due to the existence of the Anti-Spirals. If the Anti-Spirals became such an extreme threat to the Spiral Race, the Spiral Race would elevate to a greater amount of spiral energy to surpass the seemingly infinite power of the Anti-Spirals. The theoretical battle between two limitless beings would inevitably affect all other existences due to the sheer amount of infinite energy over a seemingly balanced, and limited existence of matter. This is proven when a pocket existence, created by the Anti-Spirals is completely bathed in a cosmic sized explosion of destruction, due to the battle between the two opposing forces fighting on equal grounds; possibly destroying all life in this pocket universe.
 * The Spiral Nemesis is only a theory, but the Anti-Spirals took no chances and simply acted as though it would occur factually.