|Frontiers | Pages 194-195 ||
We can thank this mysterious matter for some of nature's most beautiful displays. The collective gravity of an entire cluster can act as a powerful gravitational lens. It warps the light from more distant galaxies into ethereal arcs. Individual galaxies in the cluster also form smaller lenses, so the paths followed by light through these clusters can become quite complex. These cosmic mirages often magnify extremely remote galaxies that we would otherwise miss with our telescopes. In this way, massive gravitational lenses serve as peepholes to the distant universe.
Simulations of the cosmos with powerful supercomputers also point to dark matter's supremacy. Computers now perform so many calculations per second that astrophysicists can explore how the structure of the universe has evolved. Billions of years in the cosmos are compressed to weeks or months of computer time. Many thousands of virtual particles, representing galaxies, drift within a cube that mimics a chunk of the universe measuring hundreds of millions of light-years on a side. The programs simulate the slow but relentless effects of gravity. The attraction draws galaxies together into clumps and weblike filaments, leaving behind gaping voids. But it is the added pull of dark matter that makes these computerized patterns of galaxies resemble the patterns we see in space. Without dark matter, galaxies in the simulations simply fly apart from one another because of their own random motions.
If we have convinced you by now that dark matter exists, you probably are wondering what it is. Join the club. It's the astrophysical mystery of the century--both the twentieth century and, perhaps, the twenty-first. One researcher compared studying dark matter to assembling the pieces of an all-black puzzle in a black room. With thick black gloves on, we might add. Not only can't we see dark matter, but there doesn't seem much hope of putting our hands on it.
This is not to say that progress is impossible. The search for dark matter is happening on many fronts. For a time it seemed that much of the missing matter might be ordinary stuff like stars and planets, but too far away or too dim for us to see. We know that stars below a certain size glow faintly or fail to ignite nuclear fusion in their cores at all. We call such objects brown dwarfs. Jupiter-sized planets could wander the galaxy as well, unaffiliated with any star. Astronomers who envisioned a cloud of these things around the Milky Way dubbed them MACHOs, an aggressive acronym for "massive compact halo objects." How might we detect these ghosts? It turns out that a (continued)