More than is known is unknown. It’s because we know how much dark energy it affects the expansion of the Universe. It’s a complete mystery, besides that. It is a major mystery, however. It turns out to be about 68% dark energy in the universe. One reason for dark energy is that it is a space property. Albert Einstein realized first that empty space is nothing. The first thing Einstein has found is that more space can come into being. This form of energy would result in the expansion of the universe increasingly quickly. Sadly, no one knows why there should be a cosmological constant.
We are far more sure what is not dark matter than what we are. It’s first dark, which means we don’t see stars and planets. It’s dark. Comments show that in the universe the 27 percent required by observations is far too little visible matter. The second is that the matter, consisting of particles known as baryons, is not a dark cloud of normal matter. We know because we can detect baryonic clouds through their absorption of radiation. Thirdly, darkness is not anti-matter because we don’t see the unique gamma rays produced when anti-matter is destroyed by matter. Finally, on the basis of how many gravitational lenses we see we can rule out large black holes of galaxy size.