[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.