Science does rest on supposutions. The primary supposition of science is that phenomena are regular. Events can be studied to find predictable behavior and reproducible.
Another supposition of historial sciences is that the present is the key to the past. We got here by the operation of natural laws that remain constant over time.
These suppositions are in lieu of the competing supposition, that we were born fully formed from the head of Zeus.
Pre-suppositions are our friends. Hope this doesn't put you in over your head.