I don't understand any of this

[a completely inoffensive name]

Uhm... Last I checked that was just a rival theory.

Leading theory is that dark matter will eventually start bringing things back to the centre, no?

The 2011 Nobel Prize for Physics was given "for the discovery of the accelerating expansion of the Universe through observations of distance supernovae." As it stands right now, whatever dark matter is, it is not sufficient to shrink the universe, as we are talking an acceleration in the growth, not just that it is growing.

So, if universe was infinitely small, and it keeps expanding, does this mean there's an end to it? What's there? I'm imagining Douglas Adams as border guard...

There is a distinction made between its size and whether or not it is bounded or unbounded. Basically Earth is finite but unbounded, there is no edge of earth, you just keep walking around. I believe the most popular view right now is that the universe is both infinite and unbounded.

Fun stuff we are discovering. People seem to not grasp what is amazing about this.

Before this the furthest back we could "look" is 380,000 years after the big bang, because our "oldest" observations was the Cosmic Microwave Background. Now, with these gravitational waves we have evidence as to what happened during the inflationary period, the inflationary period was anywhere from 10^-36 to 10^-32 seconds after the big bang.

So now we have a better picture of what happened about 0.000000000000000000000000000000000001 seconds after the big bang.