Do you not believe in untested hypotheses or theorems? They are also made up without evidence.
The Big Bang itself has evidence, like the rapid expansion of the universe from the universal center in a state of decay toward entropy. According to the laws of physics, the masses that collided could not have spontaneously begun moving towards each other without force. Suggesting they began to move on their own without propulsion is just as made up as a creator pushing them.
You may not understand science as well as you think you do. There is evidence that supports the theory, but it is untested until it is repeated in a controlled experiment. According to the scientific method, the vast majority of the field of astrophysics remains untested.