"Dark dust" would, in many circumstances, have at least an infrared signature (or, rather, a signature that originates in the infrared, even if it has shifted c

"Dark dust" would, in many circumstances, have at least an infrared signature (or, rather, a signature that originates in the infrared, even if it has shifted considerably downward by the time we see it). This stuff is "dark" in the sense that the only effects we can see at all (so far, at least) are gravitational. In terms of what we think we do know, that implies mass - but mass that doesn't interact with light except insofar as it distorts spacetime.