In one new theory, it is possible that dark matter exists in a mirror universe where there are only dark versions of forces that hold the universe together. Hypothetically, dark matter is supposed to exist, even though it has managed to elude every single detection attempt so far. It might be lurking in a dark mirror universe contained within the visible universe. The merging of dark matter black holes in that realm may cause gravitational waves which can be detected. The dark mirror universe he envisions has its own versions of particles and forces that operate just like those in the universe we exist in.