The reason we call dark matter dark is not because it is shadowy material. That’s because dark matter does not interact with light. The difference is subtle, but important. Regular matter can be dark because it absorbs light, reports www.universetoday.com.
That’s why we can see, for example, the shadow of molecular clouds against the scattered stars of the Milky Way. This is possible because light and matter have a way of connecting.
Light is an electromagnetic wave and atoms contain electrically charged electrons and protons, allowing matter to emit, absorb and scatter light. Dark matter is not electrically charged. It cannot connect with light, and so when light and dark matter meet, they simply pass through each other.
All our observations suggest that dark matter and light have only gravity in common. For example, when dark matter is clustered around a galaxy, gravity can bend 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 and ordinary matter interact gravitationally. The tug of dark matter causes galaxies to come together into superclusters.
But an unanswered question is whether dark and regular matter interact only by gravity. If an atom and a dark matter particle were to cross each other, would they really just pass through each other?
Since we have not directly observed dark matter particles, we can only speculate, but most dark matter models claim that gravity is the only common link with light and regular matter.
Dark and normal matter clump around each other, but they do not collide and merge like interstellar clouds. But a new study suggests the two do interact, which could reveal subtle aspects of the mysterious stuff.
The study looks at six ultra-faint dwarf galaxies, or UFDs. They are satellite galaxies near the Milky Way that appear to have far fewer stars than their mass would suggest. This is because they consist largely of dark matter.
If regular and dark matter interact only by gravity, the distribution of stars in these small galaxies should follow a certain pattern. If dark 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-interactive model the distribution of stars should become denser in the center of the UFDs and more diffuse at the edges.
In the interacting model the star distribution should be more uniform. When they compared these models to observations of the six galaxies, they found that the interaction model fit slightly better.
So it appears that dark and ordinary matter interact beyond their gravitational pull. There is not enough data to determine the exact nature of the interaction, but the fact that there is any interaction at all is a surprise.
It means that our traditional models of dark matter are at least partially wrong. It could also point the way to new methods to directly detect dark matter. With time, we can finally solve the mystery of this dark, but not entirely invisible, material.
