“Dark matter” is stuff that we can’t see or touch, but we know it exists because it affects the motion of regular matter, which we can see. So perhaps what we perceive as dark matter is really just the gravitation from matter in another nearby universe. If dark matter and regular matter show up in more or less the same place (which they usually do), then it’s difficult to tell the difference between the existence of dark matter and a misunderstanding of how gravity works. Luckily, dark matter is not only heavy but weakly interacting; a property that reveals itself when huge blobs of ordinary matter collide and their associated blobs of dark matter notably don’t. Dark matter doesn’t interact with ordinary matter, which could be because it’s on a different brane.