
Originally Posted by
Nereid
It's a process of elimination.
If the iron and nickel were in the form of stellar mass objects, that were cold (so we couldn't see them by the light they emitted), then they'd betray their presence by lensing background objects, as they passed in front of them (quite a number of studies of this kind have been done; there are caveat of course, but such 'condensed matter objects' do not exist in sufficient quantity to account for anything more than a tiny fraction of the observed DM in the Milky Way halo).
If the Fe/Ni were in the form of a gas, it would be stonkingly obvious, as a rich spectrum of absorption lines in the spectra of anything behind the Fe/Ni. No such lines are observed.
If it were in the form of 'dust', it would also be obvious - the same physics which brings you blue skies and red sunsets would bring you 'reddening' of background objects (we don't see such reddening).
So what does that leave? Well, it leaves beach-sand sized grains up to Jupiter-sized 'planets' (actually, more like Mars-sized ones).
From here the sleuthing gets more difficult.
One strand is 'what's the (size or mass) distribution function?' The basic idea is 'no woman is an island', in this case no chunk of DM is forever isolated from everything else. When solid bodies collide, they break into many pieces, some big, some small, some tiny. When liquid bodies collide ... The various kinds of non-isolation ('interaction') produce characteristic distributions of pieces (how many bits in the form of µ-sized 'dust' compared with bits in mm-sized 'grains' compared with bits in m-sized 'rocks' compared with chunks in k-sized 'mountains' compared with ...). So, working backwards, if you don't see any gas, or dust, or grains, or Jupiters, or star-sized lumps, how can there be anything much in the way of rocks and mountains?
Another strand is 'blow-torches are vicious!'. If you set off a powerful bomb (that makes lots and lots of light), then whatever Fe/Ni is nearby, in whatever form, it's gonna be painful - vapourisation, reflection, shock waves, ... all of which would be detected (light echos, glowing gas, etc); if you aim a trillion SLACs and LHCs out into space, then whatever is in the way will light up like with astonishing brilliance; etc. When we look near such celestial fireworks, do we see the agonised screams of Fe/Ni pebbles/rocks/boulders/mountains/etc as they are seared? No.
Finally, one that I particularly like is 'close encounters of the second kind' - our solar system is moving through the local part of the Milky Way (we're like a car on a multi-lane highway where everyone is going, more or less, at 100 km/hour, and we're doing 120; only for us it's more like 40 km/s). This means that whatever 'local' dust and grains there are will also be moving through the solar system. As such particles are too big to be bothered with the extremely tenuous IPM and ISM, and not charged (so they can ignore any stray magnetic fields), some of them will 'collide' with the atoms and molecules of the upper atmosphere, be slowed, and fall as a gentle rain onto the Earth, where they will accumulate in places where nothing else much accumulates. Or where the larger of them will get burned up, and be seen as (faint) meteorites.