Pete, some things are certainly theoretical but others are a fact.

It's a fact that Stars are born, some eventually have Planets, big Stars die with a Super Nova, smaller stars die less dramatically. Those are facts that we can see happening.

It's a fact that certain elements can only exist after a Super Nova and that our Solar System is a product of that.

It's a fact that Galaxies are moving, we can observe the Red shift and also the Blue shift (Andromeda).

We can only theorise (speculate?) about the beginning and the end of the Universe though.

For our own Solar system, we can theorise about it's age and its eventual demise but we know as a fact that certain things have happened. We now know that this Planet was struck by many objects during the Heavy Bombardment but can only theorise as to exactly when that was or what the cause was.

We can only theorise as to whether early life began here from the Primordial Soup or whether the chemicals needed arrived here from "out there" by way of a Comet, Meteorite or Asteroid collision.

As to the Human race..... we can theorise as to whether we're the product of evolution or whether evolution was given a helping hand from some visitors from "out there".

Actual facts on the above two subjects are hard to come by hence theories abound.