The Inhibitors are Alastair Reynolds's answer to the Fermi Paradox in the Revelation Space novels.

The Inhibitors are the intelligence left over from a massive war - the Dawn War - that occurred between the first few civilizations that arose in the Milky Way galaxy. It is hinted in the books that they were once organic but that they later discarded their organic forms and became wholly machine. Apparently, the hints of a quadrupedal, warm-blooded vertebrate (a.k.a. mammalian) past can be faintly discerned in their architectures.

They are nonsentient - some would argue postintelligent - machinery functioning on unknown principles and capable of self-replication. They were created by the survivors of the Dawn War and their task is to inhibit the spread of intelligent life to individual planets or solar systems - the purpose, stated in Redemption Ark, being to shepherd the galaxy through a crisis 3 billion years (or 13 Galactic Turns) in the future: the collision of the Milky Way with the Andromeda Galaxy. By confining sentient life to only a few planets, they make the process of moving stars and systems (for collision avoidance during the crisis) far easier and more centralized, thus preserving life. However, when they have no choice, they tend to commit acts of xenocide in order to prevent life from spreading further.

They are, as mentioned, not sentient; however, in order to supervise and control the process of xenocide, they are capable of forming a sentient overseer from many less-than-sentient machines. They also have some very interesting and advanced technology, and apparently know about fifteen ways to kill a star - including one that allows the core material of a star to gush out and be used as a sort of solar flamethrower on planets (shown in Redemption Ark.)

They do not actively monitor the galaxy in their wait for a new starfaring culture to suppress - they plant a series of triggers near interesting phenomena or structures in the galaxy and wait for sentient life to activate those triggers. The Cerberus object around the "neutron star" Hades was one such object, and it was inadvertently activated by Dan Sylveste at the end of the book Revelation Space, thus triggering the events in the rest of the series. Another such object is the subject of Reynolds' novella, Diamond Dogs (although the nature of said object is only revealed in Turquoise Days).

In the novels it becomes apparent that the Inhibitors are starting to fail in their mission as civilisations are getting further and further into space before being found and destroyed. In fact, in the last book (Absolution Gap), in the epilogue, it is shown that humanity, with technological assistance from other starfaring (albeit hidden) cultures (e.g. the Nestbuilders), were able to push back the Inhibitors and establish a space that was Inhibitor-free. However, this introduced the problem of Greenfly, a terraforming-replicator gone wrong that ravaged systems.

Fictional species | Revelation Space