[anigbrowl]

I'm with you on this one. If antimatter was just backwards everything then there'd be large conspicuous clusters of the stuff all over the cosmos because ordinary matter would be repelling it instead of annihilating it.

While there does seem to be a lot of dark matter out there, if it were actually antimatter and had opposite gravity to normal matter one would imagine there would be a huge discrepancy between cosmologists' models and observation. The high-end supercomputer simulations and regressions cosmologists employ have good predictive power, even though we lack a full explanation for the phenomena we can observe. Cosmologists deal with such far-out concepts to begin with (by definition) that it's not as if they'd be averse to a concept like antigravity if it had predictive utility.

toys idly with desk magnets while thinking about it

Then again, given that photons are massless maybe there could be such a thing as a gravitational dipole...

/timecube

[othermaciej]

We know two things about dark matter: - It doesn't interact electromagnetically. - It does interact gravitationally and produces extra gravitational attraction beyond the electromagnetically interacting matter we know about. - It is dispersed throughout galaxies.

The first property rules out baryonic antimatter as dark matter, since that would interact electromagnetically. The last two properties rule out anything that has a repulsive gravitational interaction as dark matter.

[VladRussian]

>We know two things about dark matter:

while we don't know whether it exists at all. Whatever phenomena are explained by DM are also easy explained by molecular hydrogen (i.e H2) which is almost not detectable. [The hydrogen we know about in the space is atomic - H, and there should be orders of magnitude more of molecular one than atomic when you have atomic at such interstellar space conditions]

[InclinedPlane]

We know that dark matter cannot be baryonic matter (atoms) for two reasons. First, models of the big bang that have been confirmed by experiment closely bound the total amount of baryonic matter in the Universe. Second, dark matter doesn't seem to behave the same way that baryonic matter behaves (normal matter clumps, dark matter doesn't appear to do so).

[VladRussian]

>...models of the big bang that have been confirmed by experiment ...

to put it mildly, it is a very overreaching statement.

>Second, dark matter doesn't seem to behave the same way that baryonic matter behaves (normal matter clumps, dark matter doesn't appear to do so).

double whammy - we couldn't observe dark matter and we couldn't observe its clumps.

Or consider it another way - while it supposedly have gravitational attraction, and have no other strong interactions known, we somehow should suppose that its non-clumping, ie. gaseous/cloudy/spread-around state is still a normal thing.

[InclinedPlane]

http://en.wikipedia.org/wiki/Baryon_acoustic_oscillations

http://en.wikipedia.org/wiki/Cosmic_microwave_background_rad...

http://en.wikipedia.org/wiki/Dark_matter#Galaxy_clusters_and...