The Omegamatic (also known as the Armageddon Machine) is a giant destruction device built by the Shikadi to destroy the galaxy. It is guarded by different robots and creatures, most of them being the Shikadi themselves.
In Keen 5, it is floating close to the third planet of the Korath system (see Korath III.) According to the promotional ad found in the game help, it causes a 100,000 light year-diameter quantum explosion. Omegamatics can be bought from Vitacorp, and apparently the Shikadi are finishing the process of assembling one in Keen 5.
The Keen 5 world map basically consists of the Omegamatic and the Korath III Secret Shikadi Base. The main engine of the ship, the Quantum Explosion Dynamo, is at the very top of it. Is only reachable via an elevator, the door to which can only be opened by destroying four machines, each one protected by its own Defense tunnel.
See List of Keen 5 Levels for details on the different sections of the Omegamatic.