if so, wouldn't no one notice because everything would scale up at the same relative size and

it would seem like nothing had changed?

Well, the truth is, the universe isn't expanding. Space is. Of course, everything else in the

universe is "in" space, and ignoring the influence of any other forces, when the space between

things expands, those things will appear to be carried farther apart. But there are other

forces in the universe, and their strengths aren't directly affected when space expands.

For example, gravity and electromagnetism are able to hold atoms and solar systems together

because these forces are too strong for the slow expansion of space to overcome.

Think of it like this - if you and your cat are standing on a piece of ground that starts

to stretch horizontally between you, the ground will pull you and your cat away from each

other. But if the leash is strong enough - in particular, if it's stronger than the friction

force between your feet and the ground - it'll hold you and your cat together as space expands

beneath you.

In the real universe, it's the strength of the electromagnetic force that determines

the effective sizes of particles, atoms, and molecules so that even if the space they're

in expands, the molecules don't. Electromagnetism is basically the leash that holds atoms and

molecules together - and gravity is the leash that holds solar systems and galaxies together.

Only over super super long distances is gravity's attraction weak enough and the cumulative

expansion of all that space big enough to cause galaxies and galactic clusters to actually

grow more distant from one another over time - we know this because the light from distant

galaxies gets red-shifted as it's stretched by the expansion of the space it's traveling

through. Gravity certainly still pulls distant objects towards each other, but space expands

faster. Basically, the cat is off the leash.