The Higgs boson isn’t an atom like plutonium, it’s “further down”. I think of it in levels:
atoms, which are made up of
electrons “orbiting” the nucleus, the nucleus being made up of protons and neutrons. In turn, protons and neutrons are made up of
quarks
Quarks are a kind of elementary particle called fermions, which are at the same level as bosons (and electrons). Down here it’s all weird and quantum but in an oversimplified nutshell, it’s not so much that they physically exist as that in the maths* we can treat them as existing which makes it easier to think about.
of the physics models we use
I’m a computer scientist, not a real scientist, so I stand ready to be corrected by those more knowledgable.
edit: @SzethFriendOfNimi is more knowledgable and helped me fix this up a bit.