The non nerd answer is that there won't likely ever be a legit answer to your question. Physics will make up something that doesn't make any sense at all, the theory will gain traction and be socially accepted by others. Then, another theory will be made up to explain that one.

Absolute zero is absolute zero, quarks are not absolute zero if they are in fact something that even exists. The number value is so small for what defines their parameters that physics as we understand it right now no longer applies. The unit is so small, that it might breach space and time, be in more than one place at the same time and be both infinitely small with ceaseless parts that make it up, or it could be made up of nothing. It could break the laws of mathematics, which is what science right now says is happening, or it could be that density has a magic number value that once achieved will actually not apply to our universe. IE: its so damn small that it carries no weight in our dimension, therefore it can be made up of infinitely smaller parts. 10 is made of 8 and 9, they are made of 7 and 6, ect ect all the way down to infinity. Quarks, strings, ect ect are not the smallest part of particle, that line might be infinite.

Or, thought can transcend time and space and also break the laws of math and physics at that level. IE, God thinks it, therefore it is. Just like a PC programmer or game designer makes up complex pieces in a game world, when they are just bits of 0s and 1s, and they have no substance in the real world at all. Summed up, those pieces in the video game exist, but are not made of anything. The house in the video game is made of pixels, the pixels are made of 0 and 1s, those lines of code are made of nothing.

The same can be true for our universe and everything in it when you get down to particles that small.