Question

which particle weighs almost nothing

Answer 1

I think it's a photon. It's a particle without mass, instead of mass it's only energy.

Answer 2

But that depends, are you looking for something like you say, that weighs "ALMOST" nothing, or a particle that weighs nothing at all.

Answer 3

The neutrinos are the lightest particles in the known world, not including particles like photons which have no mass.

Answer 4

Neutrinos, electrons, gravitation, photons.