[lwat]

We know this is not the case because intersteller dust absorbs light, while the dark matter does not emit or absorb light at all. The only effect we see is the gravity.

[lloeki]

Not only just light (photons) but no other radiation as well (electromagnetic, like X or gamma rays), except gravity.

[ordinary]

Gamma rays and X-rays are photons.[0]

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[0] http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edi...