Does time actually exist? Did mankind just make it up

time doesn't exist its all made up to make life easier.
that's why time machines will never exist.

No one knows what the conditions were before the Big Bang. Keep in mind that the Big Bang is not an event that "created" the universe. It's a description of a system's behavior. In other words, we see the universe expanding and there's cosmic background radiation, hence there was some cosmic event that was likely a massive explosion.

That does not indicate this is how our universe was born. It's quite possible that this explosion process is a cycle, or that one universe triggers the iteration of another universe via these explosions.

But, if the Big Bang did in fact precede all existence, then time is a meaningless idea before any matter/forces existed. We only understand these things in relation to the idea of "time."

What time is, precisely, is still up to debate. As far as science is concerned, time is a physical dimension. In other words, it's the "thing" that allows other things to happen. As a physical quantity, it's bound to the concept of "space."

Though, if you ask a theoretical physicist, they will tell you that both space and time are largely illusions. All things are connected by a single point of existence -- what we perceive as "time" is actually the unfolding of phenomenon. So, time's not exactly a human-made concept, it's a human-based *perception*.

Here's an example.

The rate of acceleration for anything is over some unit of time squared. In other words, s^2. How exactly do we square time? We can't, really. It's just a unit rooted in whatever parameters we define. We can square a unit of time only because of mathematical rules, but in "common sense" reality this is absurd.

But, just ask any theoretical physicist: nature is pretty absurd.