I wish I could go into detail about all of the fallacies represented by this statement, but I other obligations . . . so I’ll cover a few highlights.
We can observe and measure things about the beginning of the Universe. The cosmic background radiation is one, and the fact that the galaxies are–generally–receeding away from each other, and the more distant the galaxy, the faster it is flying away from us.
You seem to have the impression that because something happened in the past . . . that it can’t be measured now.
If you believe this, then consider the obnoxious drunk who sits in the back of the bus while you sit near the front . . . and he suddenly vomits because he’s had too much to drink.
The speed of light is finite, so you actually see each part of this event after it happened in the past . . . yet you can still measure aspects about this event. You could probably estimate the volume of his vomit, and maybe make informed, intelligent conclusions about what he ate.
This is–in principle–not much different than what cosmologists and astrophysicists do when working on the physics of the Big Bang.