Comment by tremon

which doesn't "interact" with normal ("baryonic") matter

I think you mean it doesn't interact electromagnetically with either matter or radiation. It does interact with normal matter via gravity -- that's pretty much the strongest (only?) argument for its existence.

I'm not aware of any reason this object can't end up being a giant sphere

AIUI, most theories posit that solid spheres of dark matter are very unlikely because matter accretion is governed by electromagnetism in addition to gravity, and dark matter is not supposed to obey the former. Most models assume that dark matter is organized in gaseous clouds (halos); strictly speaking that's still a giant sphere, just not in the same way that Jupiter or the Sun or even the Oort Cloud is.