Hydrogen economy question! i've written my answer below but i'm missing half- equations and the advantages and disadvantages. i'm not even sure whether my answer is right at all. me with this. in advance!

Observation and experimentation have led many scientists to accept a theory about the origin of the universe. this theory is called the big bang theory. scientific evidence collected and observed by scientists around the world suggests that the universe is ever expanding from a hot and dense initial state. what makes this a scientific theory? (2 points)