The reason we call dark matter dark is not because it is some kind of shadow matter. This is because dark matter does not interact with light. The difference is subtle but important. Ordinary matter can be dark because it absorbs light. This is why, for example, we can see the shadow of molecular clouds against the background of the scattered stars of the Milky Way. This is possible because light and matter have a way of communicating.

Light is an electromagnetic wave, and atoms contain electrically charged electrons and protons, so matter can emit, absorb, and scatter light. Dark matter is not electrically charged. It has no way of connecting with light, and so when light and dark matter meet, they simply pass through each other.

All our observations show that dark matter and light have a common attraction.

For example, when dark matter collects around a galaxy, its gravitational pull can deflect light. This allows us to map the distribution of dark matter in the universe by observing how light is gravitationally lensed around it. We also know that dark matter and ordinary matter interact gravitationally. The pull of dark matter causes galaxies to come together in superclusters.

But the unanswered question is whether dark matter and ordinary matter interact. Only gravitationally If an atom and a dark matter particle intersect, can they really pass through each other?

Since we haven’t directly observed dark matter particles, we can only speculate, but most dark matter models claim that the only common link between light and ordinary matter is gravity. Dark matter and ordinary matter cluster around each other, but they do not collide or merge like interstellar clouds.

But this is new research shows that they are interactIt can reveal the subtle aspects of something mysterious. The study looks at six ultra-faint dwarf galaxies, or UFDs. These are satellite galaxies near the Milky Way and appear to have stars much less than their mass.

This is because they consist mostly of dark matter. If ordinary and dark matter interact only gravitationally, the distribution of stars in these small galaxies should follow a certain pattern. If dark matter and ordinary matter interact directly, this distribution will be skewed.

To test this, the team ran computer simulations of both scenarios. They found that in the non-interacting model the distribution of stars should be denser in the center of the UFD and more dispersed at the edges. The distribution of stars should be more uniform in the interaction model. When they compared these models to observations of six galaxies, they found that the interactive model fit slightly better.

So it appears that dark matter and ordinary matter do not interact solely through gravitational attraction. There is not enough data to determine the exact nature of the interaction, but the fact that there is any interaction at all is surprising.

This means that our traditional models of dark matter are at least partially wrong. It could also point the way to new methods to detect dark matter directly. With time, we may finally unravel the mystery of this dark but not entirely invisible material. This article was first published by Universe Today.