We have studied that anything that occupies space and has weight is matter. But the case does not hold true when it comes to the universe. In-universe any such thing is called Dark Matter. It is an object or a particle which does not seem to emit any radiation.
Scientists have said that the normal matter present in the space like the stars, planets, etc. which can be detected, makes up only for about 5% of the entire universe. But the dark matter constitutes around 27%. The rest is some mysterious energy which has been given the name of Dark Energy.
Scientists still have a doubt if this concept of dark matter is actually true. But they believe that there is something that is present in the universe that is affecting the objects present there, using its gravity. They say that the earth-orbiting telescope has picked up a new x-ray signal, which has hinted about the decaying dark matter.