Dark matter constitutes 85% of the universe. New research shows how weird this stuff is. Not only is dark matter invisible, it can pass through not only ordinary matter, but also other dark matter.
Observations by two powerful space telescopes have revealed that the mysterious stuff that makes up nearly 85 percent of the universe’s total matter is weirder than we ever thought. . . .
Dark matter’s presence is known only by its interactions with normal matter through gravity. It does not, however, interact via the electromagnetic force, which is why we cannot directly see it — it does not emit, scatter or reflect light — it is more “invisible” than “dark.”
In this new research, Harvey and his team realized just how invisible this stuff is, even to itself.
As two galactic clusters collide, the stars, gas and dark matter interact in different ways. The clouds of gas suffer drag, slow down and often stop, whereas the stars zip past one another, unless they collide — which is rare. On studying what happens to dark matter during these collisions, the researchers realized that, like stars, the colliding clouds of dark matter have little effect on one another.
Thought to be spread evenly throughout each cluster, it seems logical to assume that the clouds of dark matter would have a strong interaction — much like the colliding clouds of gas as the colliding dark matter particles should come into very close proximity. But rather than creating drag, the dark matter clouds slide through one another seamlessly.
See also this, which calls dark matter “ghostly.”