(approx.) last time I checked. Dark matter is detectable from surveys of galaxy rotation curves and galaxy cluster movement analysis - it is inferred because otherwise the rotation rates actually seen would not make sense from a gravitational standpoint from just what mass is _known_ from stars, gas, interstellar and "halo" objects - all this data from visible light (stars) plus radio telescope data (gas, molecular and otherwise such as ionized gas (single atoms stripped of electrons). Sooo, the idea is there must be "dark" matter around galaxies big and small to hold it all together - rotation rates are too fast to be explained by "normal" interacting matter, thus *something* that does not interact or is otherwise visible or detectable is there. We _only_ know about it from the mismatch between what the rotation rates should be and what is actually observed.
The dark matter is around 27% of total mass of the galaxies, clusters, everything in the universe. The unknown form (really unknown!) of dark "energy" - is 68% added to this makes 85% total mass plus dark matter = 95% mass - energy content. this leaves what we *used to* think of everything as only 5%. The Wikipedia article goes into far more detail about
how astrophysicists figured all this out - quite recently, I should add!
Anyway, now that that's as clear as mud, I'll bow out... :^)
Rick " I'm still stuck trying to wrap my brain around time-space " Jones