Most of the interstellar medium is, as far as we can tell with current technology, pretty empty and most of what is there is in the form of single atoms or tiny particles only a few molecules across. Is it plausible then to have denser regions of matter in deep interstellar space, possibly the remnants of long dead star systems? And if these areas did exist how close would we need to get with current technology to detect them?
For the purposes of giving a coherent answer assume that the areas in question are similar in volume, number of objects, spatial density, and overall mass to Jupiter's L4 Trojan Asteroids and have similarly low albedos (0.04 to 0.12). Note that for the purposes of detection these regions would be outside the Heliosphere of all the nearest stars so the amount of radiation available for them to reflect or occlude will be correspondingly low.
Note carefully that I'm not asking if we can detect something like the Trojans since obviously we can we're in the process of counting them; I'm asking how close we have to be to detect a scatter of such small dark objects when they are far from the nearest star.