Answer:
They gather data, simple as that.
Step-by-step explanation:
They ask different science communities to check their work and come to a conclusion. The universe, for instance -- which we do not quite understand, theories have helped us to get a perspective. Even though no one knows how and why the universe began, we tend to believe in the Big Bang Theory.
Read about "falsifiability" and you might understand a more!
GOOD LUCK
D.