Absence of evidence is not "proof" of the contrary, but it is evidence of the contrary.
"Proof" is a complicated word. People typically take it to mean "slam-dunk, 100%, without-a-doubt proves the case." But little in science ever works like that. Math, sure, where you can prove things analytically. But when trying to "prove" things about the universe, you have to collect bodies of evidence, and see how well that evidence matches expectations.
So, when you go out looking for something, and despite your best efforts, cannot find it, then that may not "prove" that something did not happen, but it is good evidence in support that it did not.
"Proof" is a complicated word. People typically take it to mean "slam-dunk, 100%, without-a-doubt proves the case." But little in science ever works like that. Math, sure, where you can prove things analytically. But when trying to "prove" things about the universe, you have to collect bodies of evidence, and see how well that evidence matches expectations.
So, when you go out looking for something, and despite your best efforts, cannot find it, then that may not "prove" that something did not happen, but it is good evidence in support that it did not.