What is Dark Matter?

Answer

Dear Max,

A galaxy consists of many billions of stars. Each of those stars orbits very slowly around the center of such a galaxy. Astronomers can now determine the mass of such a galaxy in two ways:

1) From the movement of a star, astronomers can calculate the total mass of the galaxy. The motion of the star is precisely determined by the total amount of matter in the system.

2) in another way : they look at how many stars there are, how much they weigh together, they also add the mass of all kinds of dust clouds and gas clouds, and thus arrive at a reliable determination of the total mass. So this is the mass of all visible matter

You would now expect that, if you determine the mass of such a system in two different ways, you should of course arrive at approximately the same result twice.

That is precisely the problem : the value of the mass that is determined in the first way, ie from the orbital motion of the star, is several times greater than the mass of the visible matter. This means that in a galaxy there is a large amount of matter that cannot be seen, about 6 times as much as the visible matter.
That is why this matter is called “dark matter”.
We do not yet know what exactly it consists of, and that is one of the great unsolved problems of modern astronomy.

So dark matter is a still unknown kind of matter that we cannot observe, but that we do know is there. We know this from the exact trajectory of the stars in the galaxy.