A hypothetical form of matter not containing baryons—that is, without protons or neutrons. An example would be the positron–electron ‘atoms’ that may constitute much of the Universe in the very distant future if protons decay. Non-baryonic matter has been suggested as a possible component of the dark matter in the Universe, for example in the form of hypothetical particles called WIMPs (weakly interacting massive particles). Neutrinos, now known to have non-zero rest mass, are also a form of non-baryonic matter, but not massive enough to account for the needed dark matter.