Spiral Nemesis

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 Antispiral acted to prevent it.

Description
As Antispiral itself explains it, the Spiral Nemesis's catalyst is the power of the Spiral running amok; being used to evolve to unnaturally greater heights in smaller periods of time, when not controlled. Antispiral 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 increase in mass throughout the universe would lead to these mega-galaxies' mass becoming significant enough to crush the entirety of time and space itself, converging all of existence to a dimensionless singularity, a Big Crunch. Antispiral's actions throughout the series are intended to prevent the abuse of Spiral Power, thereby keeping the Spiral Nemesis from ever happening.

Sharing event's of the Spiral Nemesis' creation is one of Antispiral's tactics for defeating particularly persistent enemies, being used to turn Lordgenome against his allies during Earth's first battle against Antispiral (as seen in Parallel Works episode 8). As the Lordgenome Head explains to Team Dai-Gurren later in the series, all Spiral races instinctively understand the Spiral Nemesis' cause to be true, retaining them from refuting Antispiral's reasoning. Nevertheless, Simon resolves to press onward and defeat Antispiral, vowing to do whatever he can to prevent the Spiral Nemesis.

In The Lights in the Sky are Stars, a similar event occurs within the final battle. When Super Tengen Toppa Gurren Lagann and Super Granzeboma's Giga Drill Breaks collide, Antispiral's universe, begins to collapse all mass into a singularity. Of course, this was on an order of magnitude far greater than Antispiral figured the Spiral Tribe were capable of. Regardless of the reality collapsing event both sides had induced, they had survived the Big Crunch.

Theories
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 Antispiral's theory, resolving to either prevent the event if it ever becomes a problem, Lordgenome confirms that Spiral races instinctively know of the truth when told about it, and the second movie's climax implies that it can happen with sufficiently massive amounts of Spiral Power being generated, but whether the Spiral Nemesis is truly a danger to the universe or preventable is 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 regulating the use of Spiral Power, there is no way to totally circumvent the Spiral Nemesis as long as Spiral Power exists. This appears to be Antispiral's opinion on the event, hence their efforts to limit the development of Spiral-capable beings across the universe.
 * An expansion of the previous theory postulates that the Spiral Nemesis is a natural phenomenon and 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 possible, but only because of Antispiral's efforts to prevent it. Their efforts to suppress Spiral races inevitably causes those races to rise against them; in order to surpass Antispiral's seemingly infinite power, Spiral beings would have to unleash a similarly massive amount of Spiral Power. The theoretical battle between such beings would inevitably affect all off existence due to the sheer amount of energy over a seemingly balanced existence of matter. This is proven when Antispiral's universe created by Antispiral's birth, is completely bathed in a cosmic explosion due to Simon killing Antispiral, destroying all life (if any) in Antispiral's universe.
 * 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.