Scientists estimate that everything that we see in the universe (literally) is only one fifth of the matter in the universe. We are only able to see a 20% of the total matter leaving another 80% of unknown or dark matter. It's dark because it does not interact with light at all hence making it invisible to us. But we know it's there because of the gravitational effects that we observe.
Scientists were studying the Andromeda Galaxy when they discovered a weird signal. The short wave x-ray spectrum analysis reviled an unknown pattern which could be the decay of dark matter particles.