News of the WeekAstronomy

Inventory Asks: Where Is All the Non-Dark Matter Hiding?

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Science  15 Jan 2010:
Vol. 327, Issue 5963, pp. 258
DOI: 10.1126/science.327.5963.258-b

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Astrophysicists know that 83% of the matter in the universe is dark matter—invisible stuff as yet undetected. The other 17% is detectable "baryonic matter," the atoms and ions that make up stars, planets, dust, and gas. To astronomers' surprise, the ratio of baryonic matter to dark matter seems to vary from galaxy to galaxy like the ratio of chocolate chips to dough in different batches of home-baked cookies. Now, a team announced at the American Astronomical Society meeting that they have determined that the proportion varies by scale: The largest galaxies have the highest percentage of baryonic matter, although not quite 17%; whereas the smallest galaxies have less than 1%.