The dictionary gives two definitions of eternal:

- eternal inside time. This is presentism. Requires actual infinity to exist.
- eternal outside time. This is eternalism. Does not require infinity.

So I believe the 1st is impossible whereas the 2nd is possible. — Devans99

Interesting. And do you know any rational explanation on how could anything be "eternal outside time"?

space-time can exist eternally even though time has a start — Devans99

How would you define "eternally" in this context? In other words, what is your definition of "eternal" or "eternity"?

A start of time rules out presentist models of time; it does not rule out eternalist models. — Devans99

Please advance a rational argument to prove what you assert.

You wrote: "No matter what science comes up with, like the Big Bang, one can always ask the question, and what existed before that?"

However, if we assume that time itself has a beginning, the question of what was before the beginning of time becomes irrational, hence invalid.