[seanw444]

So how do we know that these "dark matter objects" aren't actually just massive collections of normal matter that is dim enough and at such a far distance that it would appear (angular resolution-wise) to be invisible, but we can still detect the lensing?

[mr_toad]

> just massive collections of normal matter

Normal matter in the universe is mostly hydrogen, which should coalesce to form stars, which give off light. The lack of light compared to the estimated mass is precisely the paradox.

[ianburrell]

There are a few reasons. It would be visible when backlit. Gravitational lensing detection limits the size so it can't planets (MACHOs). The CMB shows that only sixth of matter interacts with other matter, the rest is only interacts gravitationally.

[dredmorbius]

Even if distinct against the cosmic background, a cloud of matter should show absorption / emission spectra, and we are very good at capturing spectography from very distant and dim objects, and we've discovered elements (helium) and determined the composition of distant objects (including the atmsopheres of exoplanets) by this method.

Light shining through dark matter, if that dark interacts with electromagnetic radiation, would show absorption lines, and I suspect they'd be of compelling interest. My understanding is that there's no observational evidence that it does. Given that we now know precisely where a dark-matter candidate is I suspect that there will be attempts made to identify any possible spectrographic signature which would confirm (if absent) or refute (if present) current understandings of dark matter's nature.