> >In this universe, not all ofthe matter around us can be readily seen. The further an object is away from usand the less luminous it is, the less visible it becomes. Just by looking at anobject is usually difficult, if not impossible, to tell the amount of mass itcontains. But astronomers have been using the measured luminosity to estimatethe luminous mass of stars, based on empirically established mass-to-lightratio which seems to be only applicable to a special class of stars---themain-sequence stars---with still considerable uncertainties. Another basic toolfor astronomers to determine the mass of a system of stars or galaxies comesfrom the study of their motion, as Newton demonstrated with his law ofgravitation, which yields the gravitational mass. Because the luminous mass canat best only represent a portion of the gravitational mass, finding theluminous mass to be different or less than the gravitational mass should not besurprising. Using such an apparent discrepancy as compelling evidence for theso-called dark matter, which has been believed to possess mysteriousnonbaryonic properties having a dominant amount in galaxies and the universe,seems to be too far a stretch when seriously examining the facts anduncertainties in the measurement techniques. In our opinion, a galaxy with startype distribution varying from its center to edge may have a mass-to-lightratio varying accordingly. With the thin-disk model computations based onmeasured rotation curves, we found that most galaxies have a typical massdensity profile that peaks at the galactic center and decreases rapidly within~ 5% of the cut-off radius and then declines nearly exponentially toward theedge. The predicted mass density in the Galactic disk is reasonably within thereported range of that observed in interstellar medium. This leads us tobelieve that ordinary baryonic matter can be sufficient for supporting theobserved galactic rotation curves; speculation of large amount of non-baryonicmatter may be based on an ill-conceived discrepancy between gravitational massand luminous mass which appears to be unjustified.
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