Theoretical physicists crunching the maths have established that an exotic particle, called the sexaquark, consisting of six quarks – two ups, two downs and two stranges, could turn out to be very stable.
Familiar matter – protons and neutrons – are comprised of 3 quarks each and are stable. Particle accelerators have managed to produce 2-quark and 5-quark particles – but these are unstable and quickly decay. It just so happens that the sexaquark, if it has low mass, would be very stable. And low mass particles are not generally produced in colliders that operate at much higher energies, which is why it may not have been seen so far.
What is really exciting about this idea is that it is possible that during the early universe an abundance of these sexaquarks may have been produced, and being very stable they might still exist, not decaying, not interacting with normal matter or photons, except through gravity – in other words ‘dark matter’!
Update 6th March 2020: A further study today (calling the paticle a d-star hexaquark rather than sexaquark!) suggest that the hexaquark would be stable within a neutron star and probably within a Bose Einstein Condensate (BEC – this is an exotic state of matter that can form close to absolute zero temperature wherein a collection of particles behaves as a single particle); the important point being that the hexaquarks could have formed in a a BEC favoured by the very early conditions of the universe – hence long-lived dark matter. The researchers intend to experiment to see if they can form a BEC of hexaquarks, and will also look for characteristic (though rare) gamma-ray decay signatures of neutral d-star hexaquarks.
Research publication (Physics G letters)