I see where this is diverging a little bit.
But everything is expanding. Including matter. But the mass isn’t chaning.
But this also includes the space in between the objects.
So objects are getting further apart, but so are the objects getting bigger at the same rate.
The mind bend for me was realsing it’s not space that expanding really, it’s distance.
This is why distant light is red shifted. Because what started out as white, has had the wavelength expand with the universe, making it appear more red.
So yes attoms are expanding. everything is expanding. I mean that very literally.
Let me put it this way.
If you had a million year old meter stick. It would always be a meter. Accurate to the definition of a meter using the wavelength of I don’t remember what off the top of my head. It would always be a meter exactly.
But.
If you magically placed the meter stick next to itself from a million years ago, they would not measure the same. Even though they started with the same definition.
Like I said. Space isn’t expanding. Distance is.
EDIT I don’t mean the distance between things is expanding. The definition of what a distance was is expanding. So yes, attoms, when measured by size (the distance from one edge to another) has also expanded.
But in the same breath, the measured distance never changes. Because the way you use to measure distance has also expanded by the same amount. So nothing ever changes in reality, but everything is just constantly bigger.
Physics is full of hard to explain paradoxes.