First step is google the evidence for the standard model (Big Bang). Come back with a summary.
Then…do you insist it began whole (like I do) so that every moment is a kind of beginning, and the whole is “eternal”? Do you think the whole, or each moment, began itself?
If not, do you think the eternal being with which the whole is co-eternal has zero power, &/or zero (personal) will, to initiate the universe (as a whole, or each moment)?
What is your best explanation, if not an eternal personhood with the ability to initiate and subsume the universe in their being? Why would it stop existing?
And yet… here’s the universe. Please summarize the evidence for its beginning after you google it. Or use Bing.
If the event is physical, it’s part of the universe after it already began.