Why Think There's Dark Matter?

Most recent answer: 03/29/2015

Q:
Hi,Could Dark Matter be nothing more then us not accounting for the center mass of galaxies and clusters?From my understanding when we calculate the mass of a Galaxy (or a cluster) it's only 10%-15% of what it should be, thus it is missing something. Is it just missing the mass in the center? Are we taking that into consideration? If so can we even calculate the mass of a black hole?Thank you.Ed
- Ed (age 42)
Sun City, AZ, USA
A:

That's all part of the calculations already. You can figure the mass of the central black holes from the rotation rates of stars near the center. The overall pattern of rotation rates all the way out looks like it's influenced by a big blobby mass, attributed to dark matter. We think this dark matter interacts very weakly with our type of matter and even with other dark matter, because when galaxies collide the dark matter keeps going while the visible matter gets slowed down by the collision. That shows up in the behavior afterward, with the extra gravitational mass no longer centered in the same places as the visible matter. Here's a nice picture of the effect:

 

MACS J0025.4-1222" by NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) - http://hubblesite.org/newscenter/archive/releases/2008/32/

There are other lines of evidence for dark matter, involving the cosmic microwave background radiation, but those are less direct. If, however, no actual dark matter particles are found in more sensitive searches, the whole picture may have to be re-thought. There are alternative ideas about modifying the law of gravity, but that seems like a particularly ugly alternative.

Mike W.


(published on 03/29/2015)