Greg Kochanski

If a large cluster of galaxies (here CL0024+1654) exhibits strong lensing, the mass of the cluster warps space enough for the same background galaxy to be visible along several lines of sight. <object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/byXL7707Ctc&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/byXL7707Ctc&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

Gravitational lensing gives us our only direct window into the dominant matter component of the universe, dark matter. Other than the fact that it has mass, we know only negative properties of dark matter: it is not baryonic, it has undetectably small interactions with normal matter, and so forth. Consequently, detailed maps of dark matter distributions are currently the most realistic hope of making some positive statements about the dark 95% of the universe. Unless we understand the properties of the dark matter and dark energy that forms most of the universe, we have little hope of a detailed understanding of our origins.