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. Despite never occurring in the series proper, it serves as the driving force of the entire series, as the Anti-Spiral acted to prevent it.

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. The Anti-Spiral theorized that Spiral Power would eventually reach a point beyond lifeforms' ability to control, causing them to spontaneously evolve into full-sized galaxies. The sudden imbalance of mass in the universe would lead to these mega-galaxies devouring each other; if enough were formed across the cosmos, the resulting interactions would form a supermassive black hole that 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.

Sharing the theory of the Spiral Nemesis is one of the Anti-Spiral's tactics for defeating particularly persistent enemies, being used to turn Lordgenome against his allies during Earth's first battle against the Anti-Spirals (as seen in Parallel Works episode 8). As the Lordgenome bio-computer explains to Team Dai-Gurren later in the series, all Spiral races instinctively know of the possibility of the Spiral Nemesis, preventing them from refuting the Anti-Spiral's theories out of hand. Nevertheless, Simon resolves to press onward and defeat the Anti-Spiral, vowing to do whatever he can to prevent the Spiral Nemesis if it should indeed threaten to become a reality.

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 (which contains an infinite multiverse) 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; in other words, while still theoretically possible, the Spiral Nemesis 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, and Lordgenome confirms that Spiral races instinctively know of the possibility of the event, but whether or not the Spiral Nemesis is a real danger is never definitively answered, being left up to the viewer's interpretation.

Among the fandom, several theories about the Spiral Nemesis have been proposed:
 * The Spiral Nemesis is the result of an extreme overuse of Spiral Power; thus, it can be prevented if Spiral Power is used responsibly and with restraint. This seems to be the most common theory, as well as the one that Team Dai-Gurren believes, given Simon's refusal to abuse Spiral power to revive Nia Teppelin or his other fallen comrades.
 * 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. Although it can be delayed by limiting Spiral Power, there is no way to totally circumvent the Spiral Nemesis.
 * An expansion of the previous theory postulates that the Spiral Nemesis is not just inevitable, but a necessary part of the universal life cycle, similar to the concept of a "Big Crunch". 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 and their desire to suppress all Spiral Power. To surpass their seemingly infinite power, Spiral beings would have to unleash a similarly massive amount of Spiral Power; 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 the Super Spiral Space 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 the Super Spiral Space.
 * Similarly, the Spiral Nemesis could theoretically occur as the result of a war between super-advanced Spiral races, as a result of them drawing on ever-increasing amounts of Spiral Power to surpass each other. It is implied that this scenario occurred in the enigmatic prologue of Episode 1 (Where Simon says that "All the lights in the sky are our enemies"), and preventing this outcome is why Rossiu spent the 20-year timeskip in Episode 27 promoting peace and cooperation between Spiral races across the galaxy.
 * The Spiral Nemesis is only a theory, based on the logical extrapolation of the capabilities of Spiral Power and the consequences of overusing it. The Anti-Spirals simply acted as though it would occur factually, whether because they truly believed it to be inevitable or because they didn't want to take any chances.

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.