Expert: Jayendra Upadhye - 8/5/2006

Question
What is dark matter exactly? And what is the difference between comets and meterites? Do most comets orbit stuff, but rarely hit anything? I know people talk about Hailey's comet from time to time and having witnessed it so I assume it is still alive and well. lol.

Answer
Hi james!
Dark matter is precisely that! Dark!
Meaning it is an all inclusive term covering all matter that is NOT luminous.

Normally when looking at the larger picture, one tends to look at all mass that is "luminous" ..meaning mainly stellar mass.
Afterall the sun is "all the mass there is" if one were to get down to the final balance sheet of mass in the solar system.

But we (non luminous earth guys) would still be there...though infinitesimal compared to the sun.

But while looking at the bigger picture astronomers realised that the rotation of stars around the milky way (our sun doing that every 225 million years..), could be explained only if there was 10 TIMES more mass in the spiral plane of the galaxy than was acounted for by luminous matter (stars).

That started this whole Drak matter debate.
Perhaps there are huge number of brown and infra-red dwarf stars and supergiant gas planets (hot jupiters and cold jupiters), as well as millions of rogue planets going around after their progenitors were disturbed by impish binary companions!
we are haunted by one such companion who upsets the oort cloud every 65 million years and sends cascades of comets careening inwards into the inner solar system. (Nemisis hypothesis).

Most comets have hyperbolic paths and they leave the system never to return.
Only a small minority develops periodicity. (halleys is one).
To my knowledge, in its last pass, halleys broke up into smaller fragments. But i could be wrong. check it out on the web.
The supernovas seed the galaxy with dust comprising of solid oxides and salts which form immediately after the supernova cools and crosses the respective recombination temperatures. Thus hydrogen and oxygen "BURN" and form vast clouds of Water! Likewise Sodium and the halogens would form salt particles! all that would eventually form planets and get locked away as dark matter.

That is how earth got all of its water in the first place!
And mineral oxides and sulphides and Silicon dioxide that makes its mantle!
And the compounds of carbon that make "us".
As Sagan said.."we are star stuff"!
Lastly dont forget the Black holes!
Jayen