Hmm... I'm a passive DM sceptic, "exotic" DM anyway. Not bigoted you understand, just not persuaded by all the handwaving I have been exposed to in the last few decades.

However,  the figures I read were a good deal more than a mere trebling of stars could correct for. I seem to remember something like >90%

Try me again when they treble the stars again.

Anyway, the trebling is likely to be biased in favour of an unexpected number of dim (read: light) stars, so the mass increase would be less than 3X.

Hear the expert!  :-)

I'm like yourself Jon - I remain sceptical until there's a bit more evidence. Dark matter (so far) sounds a bit too convenient for me.

I think your 90% figure might include dark energy. I reckon a trebling of stars (and associated planets, if I heard correctly) should just about account for DM. Beyond that....  I was hoping an expert would drop by and enlighten us (forgive the awful pun).

If there are many more planets than expected, would that account for a good deal of mass, e.g. lots of iron etc.??

My knowlege and understanding is very limited but here is what I understand the numbers to be. Because we cannot see all of the universe or even know how big it might be, we have to talk in terms of energy density and matter density. In terms of overall matter/energy density dark matter accounts for 23% of the total, normal matter 4.6% and dark energy the rest. This means that if we look at just matter density it is about 20% / 80% split between normal and dark matter.

As a matter of interest, the idea of dark matter has been around  since the 1920s or 1930s.

I can believe that matter exists that does not contain electrons and indead has no electrical charge. This would account for no observable interaction with normal matter. If this matter pushed instead of pulled in gravitational terms then I could see it causing the universe to expand. Over long periods it would be reasonable that this matter would be ejected from within solar systems so that we would see no evidence of expansion within our own solar system. It might also be ejected or squeezed out from galaxies. So, existing outside large structures, it would tend to push them together and away from each other. Assuming that this matter can interact gravitationaly with photons and that its density is not the same everywhere, we sould be able to spot something like gravitational lensing happening away from large structures. I expect that my thinking is flawed but at least I am thinking.