I'm an IT consultant who's been reading New Scientist for 30 years, not an Astrophysicist! Now, you mention "matter ... sparse enough ... massive". I think yo

I'm an IT consultant who's been reading New Scientist for 30 years, not an Astrophysicist! Now, you mention "matter ... sparse enough ... massive". I think you have answered your own question. The matter in question may be sparse but if there is enough of it, then it will become "massive" and hence detectible. Not sure what a "cold electron" is - it sounds unlikely. Dark Matter is postulated and not a "thing" per se. A lot of good science does seem to imply that 4/5ths of matter is unseen. The key thing I am trying to get across is that our human need to slap a name on something is not appropriate here but we do it anyway. The cool kids with massive budgets and impressive machines in space cannot yet tell you and I what the stuff is but for now we will call it "dark matter". It might sound a bit mad for people more used to dealing with certainties like Civil Engineering (lol - Britannia Bridge in London - resonance). I'm a Civ Eng grad. That's quite a long way of saying "no"!