You can do this at home
Take a piece of string about three feet long and attach a lead weight
Now swing the lead weight in a circle above your head.As you increase the speed the tension in the string goes up very quickly (double the speed and the tension goes up by a factor of four). Eventually the string will break or you won’t be able to hold on to it any more.The lead weight will fly off at a tangent.
Gravity and Orbits
When an object is in orbit gravity replaces the string so that above a certain speed the object will escape the pull of gravity. (This is called the escape velocity).The force of gravity depends on the mass of everything about which the object is orbiting.
When you do all of the sums you find that there is not enough mass is the galaxy .The galaxy should really being flying apart and it isn’t.
In fact there is only 20% of the required mass which is visible to us.
This is a big error and the law of gravity can’t be wrong (can it ?). So scientist have invented something called “dark matter”. Dark matter is very strange .It has mass but you can’t see it. And that is not because it is a long way off in a dark place it’s because it has mass and you can’t see it.80% of the universe is made up of dark matter or as some people think there is something wrong with the law of gravity.