How do we know that dark matter is collisionless?

		We have explored its properties in the laboratory and particle accelerators.
		Galaxy cluster collisions show that dark matter follows the hot X-ray emitting gas, which is collisionless.
		The rapid motions of galaxies in rich clusters tell that dark matter is collisionless.
		Galaxy cluster collisions show that dark matter follows the stars themselves, which are collisionless.
		The flat rotation curve of spiral galaxies indicate that dark matter is collisionless.