No one really knows the answer to this. Scientist can explain how a big bang lead to the universe we have today but we can’t explain what caused it.
One idea is that after the big bang the universe expands, like it is now. After a very long time it might start to shrink again as the energy from the big bang dies away and gravity starts to pull objects to together. Over a very long time billions of billions of years the universe will get smaller untill all matter is compressed to one point. This build up of mass and energy would then cause another big bang and the process repeats again and again.
However even if this is true we still don’t know what caused the first big bang.