What is one of the primary unanswered questions about the origin of the universe? what type of elements formed first? approximately how long ago did the big bang occur? how long did it take the earth to form? what came before the big bang?

last option, what came before the big bang?

Explanation:

The big bang theory states that the universe started as a dense nucleus of matter: a huge amount of matter concentrated in a tiny spot.

This is the conclusion of equations and evidences that prove that the universe has been and continuous to expand: since it has been expanding, there was a moment when it was as small and dense as it is possible.

So, the expansion is the result of violent explosion.

The time during which the expansion has been happening (this is how long ago the big bang occured) has been estimated thanks the the observation of the speed of recesion of the galaxies, but nothing can be told about what came before the bing bang occured.