We believe that DM exists in the form of a particle- as does the ordinary matter with which we are familiar (electrons, neutrons, protons, and the large array of other particles that make up the Standard Model of Particle Physics).
Each of the particles in the Standard Model has its own particular properties that define what it is, and how it interacts with the other particles. And so it is for DM; however, we do not know what all its defining properties actually are…
One of the most basic properties for any particle is its mass. We know, on a cosmic scale, what proportion of the universe’s mass-energy exists in the form of DM. And from observations and simulations, we can infer and predict the distribution of DM throughout the universe- where it should be most concentrated, and where it should be less dense, for example.
But as to the mass of an individual DM particle? Well, we simply do not have a definitive answer. Like a Schrödinger’s Cat of a question, this is both annoying and exciting at the same time. Annoying because we don’t know the answer, but exciting because we don’t know the answer, and our curioisty is piqued.
Theoretically, there is a vast range of masses over which DM particles could exist. Various considerations restrict this range to a smaller- but still very extended- range of masses. Experiments such as SABRE South are looking for WIMPs (Weakly Interacting Massive Particles), which covers the mass range ~ 7.1 x 10-27 kg to 1.78 x 10-24 kg in mass units, or around 4 GeV to 1,000 GeV in terms of energy units.
Other experiments (such as ORGAN at the University of Western Australia), are looking for WISPs (Weakly Interacting Sub-eV Particles), with masses ~ 3.56 x 10-42 kg to 3.56 x 10-40 kg in terms of mass units, or 2 x ~ 10-6 eV to 2 x 10-4 eV in energy units.
This is a variation of ~1016 between the high-mass and low-mass ends of the scale- quite a difference indeed!
As work with various experiments around the world proceeds, various sections of these mass ranges are tested and excluded, but much work remains to be done.
You may have noticed the words “weakly interacting” above. This defines one of the known properties of any candidate DM particle- that it must interact only very weakly with other particles.
This doesn’t mean no interaction; if that were so, there’d be no point in building direct-detection experiments. It’s just that the interaction is rare. We expect this because we know there is a lot of DM about, but we simply don’t see it interacting that much with other forms of matter.
It does interact gravitationally with ordinary matter- that’s one of the key pieces of evidence for it. But other modes of interaction are very limited, and must be searched for with extraordinary care and precision, and that is what the various direct-detection experiments (SABRE South included) are doing.
So- we have an invisible particle, that interacts with ordinary matter via gravity, and very rarely by any other means, with a mass that covers a huge range of possibilities, but with sections of that range being whittled down as research progresses.
It is these unkowns, and the importance of eventually finding out what DM actually is, that makes the search so exciting.