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

They didn’t. Until roughly 380,000 years after the Big Bang, the entire universe was a thick opaque cloud of plasma of electrons and nuclei.

As the universe expanded, it cooled off enough to let the plasma become atoms, and the cosmos became transparent.

It is space itself that’s expanding, not the particles.

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

This is a common misconception. A supernova is the explosion of a particular size star that occupies a point in space. The Big Bang was not an explosion, but the expansion of space.

Imagine infinite points packed together with infinite density. Those points occupy everywhere. There is no inside or outside.

Now imagine each of those infinite points moving further from its adjacent points, with the space between them expanding at an increasing rate.

For 380,000 years, the universe was entirely comprised of plasma but then it cooled enough that the electrons and nuclei combined to form atoms, molecules, dust, stars, planets and so on.

We have evidence of the above (post-380k years) since the discovery of the cosmic microwave background radiation (CMB).

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

The best current hypothesis is Dark Energy, but we can only measure its effect. It falls outside the known laws of physics.

Edit: some relevant reading

> 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