[skohan]

I thought "dark matter" literally refers to any matter which is not emitting light, and the speculation about there being some "spooky" form of dark matter is only to account for the sheer quantity which must exist but we can't observe

[zuminator]

One of the odd characteristics of dark matter, aside from being dark, is that it forms halos around galaxies, but within those halos the dark matter is distributed evenly; it doesn't collapse into clumpy matter. Stars and planets, on the other hand, clump and cluster. If there were sufficient numbers of rogue planets to account for dark matter, there would have to be thousands or millions of rogue planets for every star that we can see. At that density they couldn't stay hidden -- over cosmic timescales they would be attracting each other, forming clumps that would partially occlude stars, and turning into stellar nurseries with detectable frequency.