where does dark matter hide ?

I'm much interested in the so-called dark matter, though about which i know only a little. It does not interact with light. Neutrino does not interact with light as either. But neutrinos can be detected by, e.g., weak interaction. But dark matter even does not lend itself to weak interaction. Nevertheless, it is supposed to engage in gravitational force, which is exactly why it is stipulated. I'm even not sure if this matter really exists, or it is just that we need new theory. Dark matter is required for the present physical theory, but perhaps dispensable with a new theory. Anyway, it is theory, it is our language, that decides what can be observed and what we can speak of.

After all, a recent work suggests that, large galaxy hides more dark matter that small ones, seeming a result of gravitational attraction.

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 led by Stacy McGaugh at the University of Maryland, College Park, has 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%.

McGaugh and colleagues compiled the ratios for more than 100 galaxies ranging from supermassive ones to dwarfs. Researchers infer the amount of dark matter in a galaxy from the motion of its stars. They estimate its baryonic mass from the amount of light the galaxy emits, which can be converted to the total mass of its stars, and a measure of atomic hydrogen in the galaxy, which provides an estimate of the interstellar gas.

"What we find is that there is a very systematic variation in the ratio with scale," says McGaugh, who presented the findings at the American Astronomical Society meeting in Washington, D.C., last week.^* "When you go to the very large galaxies, the baryonic matter can be as much as 14%. As you go down in size, you see that galaxies fall short of the cosmic fraction [17:83] by an ever-increasing amount." In galaxies the size of the Milky Way, "all the stars and gas add up to only a third of the baryonic matter you would expect," which is about 5%. And in the smallest dwarfs, baryonic matter is a hundredth of what's expected—as minuscule as 0.2%. "These are very interesting results" that quantify the "missing baryonic matter problem," says Joel Bregman, an astronomer at the University of Michigan, Ann Arbor.

Where is all the missing baryonic matter lurking? One hypothesis is that its particles are interspersed within the galaxy's dark matter halo in the form of undetectable hot gas. Another is that supernova explosions have blown it into intergalactic space. This second idea would square with McGaugh's findings: Large galaxies, with stronger gravitational pulls, would be able to retain more of their baryonic matter, whereas smaller galaxies would let more escape. But so far, McGaugh says, that explanation is just one of several lines of speculation.