> But matter can't be created nor destroyed right?

Sure it can, nuclear reactors turn mass into energy - or is it even matter if the released energy is actually the binding energy of a nucleus which appears as mass?

Energy can't be created or destroyed though (except by the big bang apparently), only transformed from one form to another.

> How did small atoms expand themselves to become size of galaxies.

They didn't start off as atoms, the Universe was far too hot for atoms to form for about 380,000 years - and the CMB is the remains of the light that was flying around at the moment when things were cool enough that atoms could form and the universe became transparent.

If you're wondering why it didn't all collapse into black holes at that density, we think that might be where the black holes at the center of galaxies came from…

Also, the expansion doesn't have a border or edge, it's more useful to imagine new empty space being injected everywhere all at once, like infinite raisin bread rising.

> I thought big bang was like a supernova where a massive amount of matter exploded

Nope, it's a time-like surface from which energy and spacetime poured forth, and could reasonably be described as a white hole - you can draw a ray in literally any direction you like, and it'll eventually intersect the big bang at the moment of our universe's creation.

> which external force was applied on universe to stretch it?

We have no compelling evidence for anything outside our 3+1 perceivable dimensions, and what we can see is same-ish in every direction for as far as we can see, so why not an internal force?

It's called dark energy fwiw, and it's an ongoing field of study since we know almost nothing about it