We know. Because we look at things and we see them. It may sound simple, but it is. This is very deep. We can see everything that is made of atoms because we can see it because light bounces back at it. When you look into space, you see stars and constellations. Some are as beautiful as we live. It has the shape of a snail, and it orbits beautifully in space. In the 1930s, scientists began to measure the motion of galaxy clusters and calculate the amount of matter involved. I found that in these groups, it was not enough to gather them. This mysterious result Later galaxies were measured and confirmed.
What you see in galaxies is not enough to attract enough gravity to collect them. So what we see should be wasted, but this is not the case. There must be something we cannot see. This is called Dark Matter. Today The best proof for Dark Matter is the measurement of the Big Bang, the Cosmic Microwave Background CMB. But this is a different story. All the evidence we have now shows that they have Dark Matter. And it is believed that there are many of these beautiful snail-shaped galaxies in the sky.