Think of the expansion of the universe as a superlatively weak repulsive gravitational force that scales with the distance to an object. It is so weak at short scales that it is more weak than attractive gravity by a greater factor than attractive gravity is weaker than electromagnetism. It is so small at short scales that even the normal attraction of gravity is so much stronger as to make it undetectable.

However, at scales so large that gravitational attraction has attenuated to nothing, this repulsive component of gravity is still getting stronger. At the scale of galaxy clusters, it is pushing everything in the universe apart in all directions: faster the farther apart they are. At a certain distance, it becomes so strong that objects will appear to be moving away from you faster than the speed of light.

The distance at which that happens is called your Cosmological Horizon, and it has similarities to the event horizon of a black hole turned inside out. Unlike a black hole, however, there isn't a single unique event horizon. Every distinct gravitationally-bound object (galaxy cluster) in the universe has its own cosmological horizon; every other object is being pushed out towards it, and vice versa.

(I am not a physicist) As far as I understand the current theory (i.e. our best guess), the 4 fundamental forces are somehow countering the expansion so that, even as everything in the universe expands, including e.g. every single atom, the forces are even stronger and are pulling stuff together.

I think this just opens more questions than it resolves, and would really like to hear a better explanation.

No. Just because space is expanding doesn't change the electromagnetic forces holding you together.

But just thinking of the electromagnetic forces in terms of what is required from them to keep matter together in the context of that same matter effectively expanding faster than the speed of light is... at best weird.

The speed of expansion depends on the size of the object. Human-scale things aren't expanding at the speed of light; you need to get to larger-than-galaxy-cluster scales for that to happen.

(In fact space inside galaxies isn't expanding at all, only space between galaxies. But I digress.)

It's not faster than light at the scale where there's matter in the universe. Across a planet or solar system it's not measurable.

It's when you measure between one side of the observable universe to the oposite side that the expansion speeds get crazy.

> Doesn’t that imply that we are getting “bigger”?

Yes, if by "we" you means "large scale cosmological structures".

But galaxies stay roughly constant size because gravity holds them together. Smaller things are also held together by various forces.