Heading towards the Winter solstice the days are getting shorter here in the Northern hemisphere. It’s a time when our imaginations dwell on stories of dark nights and the mysteries they might hide, but some physicists spend the whole year searching for the mysteries of the dark.
We know that everything we see in the night sky is only a small amount of the matter that is really there. The ordinary matter that we see appears to account for only a sixth of the true amount of matter in the universe. The additional unseen mass is not visible directly to any telescope, but we can measure the effects of it on the motion of stars and galaxies and by other gravitational effects. No one yet knows with certainty what this Dark Matter is made of.
Dozens of experiments around the world are involved in trying to identify the nature of Dark Matter, and many different candidates that have been proposed.
Some of the best known of these experiments look for particles called WIMPs (Weakly Interacting Massive Particles), and others look for much less heavy particles called WISPs (Weakly Interacting Slim Particles). Of these WISPs, the two most popular to search for are the 'axion' and the 'dark photon'.
Both axions and dark photons have a very interesting property; under the right conditions, they transform into light. Searching for these particles often means looking in sealed boxes to see if any light appears inside or on the far side of a wall to see if the light has shone through.
Of course, in reality, these walls and boxes are parts of complicated experiments, and the amounts of light being searched for are far smaller than anything that could be detected by the human eye. Nevertheless, if axions or dark photons exist then even in the darkest of places a little light may be coming from the mysterious Dark Matter.
It’s a comforting thought on a dark night.