How do we know protons, neutrons and electrons exist?

How do we know protons, neutrons and electrons exist? And what exactly is a positive or negative charge?... meaning what is it that makes it positive or negative.

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K^2
We know that there are two types of electric charges. Which one you call positive and which one you call negative is purely a convention. There are some symmetries in particle physics that tell you that there really are exactly two types of electric charges.

In contrast, there are 6 types of color charges, which are responsible for nuclear forces. They are conventionally called red, green, blue, anti-red, anti-green, and anti-blue. These are just names, though. No relation to visible colors. So there is no fundamental reason why there should be just "positive" and "negative" electric charge, at least none that we see so far, but there are exactly two.

What actually causes charge, and why it can only change by whole numbers is still unknown. Fractional charge of quarks is also a big mystery so far. They can still only change their charge by 1, but it is a flip between +2/3 and -1/3, which is a bit strange.

thank you Sakha. And K^2 sorry but you confused me. I basically mean what is a charge? what is it about the object that gives off this thing that we call charge.

K^2
That's what we don't know.

I basically mean what is a charge? what is it about the object that gives off this thing that we call charge.
I think it's helpful to visualize these patterns:

Mass and charge are two basic properties of matter.

Mass - all matter has it. Charge - some matter has it and some doesn't.

Mass - there is just one kind, so it always has a positive sign. Charge -there are two kinds, so we give it positive and negative signs.

Due to mass, gravitational force always attracts. Due to charge, electric force sometimes attracts and sometimes repels.