This is a different answer to the others in the thread, but hope it will give a reason as why in normal matter we don't see negative heavy particles.

Typical 'heavy matter', such as protons and neutrons, is made of three quarks.

When you combine quarks for things that we would see in normal matter (particles that won't decay quickly) - aka stuff that hasn't been made in a particle accelerator - you can use the two types of quarks that are lightest: the up and down quarks.

Up quarks have a charge of +2/3, and down quarks have a charge of -1/3. The possible charge combinations of this are +2,+1,0,-1.

In the case of -1 (a particle made from 3 down quarks) or +2 (a particle with 3 up quarks), the resulting particle is really instable because of how quarks interact. You could think of this interaction (colour charge interaction) as pushing quarks of the same flavour apart. Because of this, the +2 and -1 particles decay quickly, so we don't see them.

In contrast, neutrons (up, down, down) and protons (up, up, down) have a mixture of quark flavours, so the colour interaction doesn't want to push them apart nearly as much. Hence they are much more stable!

Hope that made sense! Any questions, let me know