Spiral Nemesis

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

Description
As the Anti-Groove itself explains it, the Groove Nemesis's catalyst is Groove 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 Groove Power was to be generated, all Groove 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-Groove's actions throughout the series are intended to destroy all Groove Power in the universe, thereby keeping the Groove Nemesis from ever happening.

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

In The Lights in the Sky are Stars, it is shown that the Groove Nemesis is more than just a theory. When Super Tengen Toppa Gurren Lagann and Super Granzeboma's Giga Drill Breaks collide, the entire Anti-Groove 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-Groove thought the Groove races were capable of. This shows that the Groove Nemesis, while still theoretically possible, requires a great deal more mass and Groove Power than even the Anti-Groove had initially surmised.

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

Among the fandom, several theories about the Groove Nemesis have been proposed: Each of these theories - as well as others - have their believers and detractors, but the true nature of the Groove Nemesis is and likely always will be a mystery.
 * The Groove Nemesis is indeed a real threat, but if Groove 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 Groove Nemesis is an inevitable consequence of evolution; when the universe's life forms grow too advanced, their combined Groove Power will destroy everything. Limiting Groove Power certainly delays it, but there is no way to totally circumvent the Groove Nemesis.
 * The Groove 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 Groove Nemesis is a possible outcome due to the existence of the Anti-Grooves. If the Anti-Grooves became such an extreme threat to the Groove Race, the Groove Race would elevate to a greater amount of Groove energy to surpass the seemingly infinite power of the Anti-Grooves. 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 Groove Space, created by the Anti-Grooves 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 Groove Space.
 * The Groove Nemesis is only a theory, but the Anti-Grooves took no chances and simply acted as though it would occur factually.