Where is most of the universe? It appears to not be in the form of visible matter in stars, or even in planets orbiting them. But huge quantities are out there, and its collective mass and gravity helps determine the evolution of galaxies and the fate of the universe. Here we explore how the gravity of dark matter, in whatever form, is detected by the lensing of light passing close to it from more distant, brilliant young galaxies and quasars, a concept first suggested by Einstein and tested with great success in a total solar eclipse in 1919.

1			Compare this interactive with the one on eclipsing binary stars; in what way are they similar? Need a Hint?
			A)	Just as stars are bound in pairs, so binary pairs of galaxies also exist and can sometimes pass in front of each other.
			B)	Both eclipsing binaries and gravity lenses depend upon almost perfect alignments as seen from Earth.
			C)	The gravity lens can alter the image and brightness of the more distant member of the pair during the eclipse.
			D)	The two interactives are in no way related.

Here we explore how the gravity of dark matter, in whatever form, is detected by the lensing of light passing close to it from more distant, brilliant young galaxies and quasars, a concept first suggested by Einstein and tested with great success in a total solar eclipse in 1919. But how do we know we are in fact seeing this?

2			What convinced scientists that the image of the double quasar was in fact a gravity lens, and not two unrelated quasars appearing close together due to change alignment? Need a Hint?
			A)	The two quasar images were exactly midway between the galaxy doing the lensing.
			B)	The two images were exactly the same brightness.
			C)	The two images showed exactly the same red shift and chemical composition.
			D)	It was never been showed that indeed there was only one quasar involved.

In this interactive we saw how the gravity of a massive object is detected by the lensing of light passing close to it from more distant quasar. Bur what if the object being lensed is not a point-like quasar image, but an extended galaxy?

3			With the Hubble Space Telescope, in several cases we have found that massive nearby galaxy clusters allow us to see out to distances of 12-13 billion light years, looking at distant very young galaxies. How do they appear? Need a Hint?
			A)	They are multiple points of light, like Einstein's Cross, instead of binary pairs.
			B)	The distant galaxies appear as reddish arcs, brighter than expected for their vast distances.
			C)	The distant galaxies are blue shifted by the lensing.
			D)	The galaxies are made fainter by the spreading of their light.

Here we explore how the gravity of dark matter, in whatever form, is detected by the lensing of light passing close to it from more distant, brilliant young galaxies and quasars, a concept first suggested by Einstein and tested with great success in a total solar eclipse in 1919. We find that the viewing angle plays a huge role in how the see the distant object here on Earth.

4			Einstein's Cross is (G2237 +0305) is a famous gravity lens imaged by the HST, where the central cloverleaf is in fact four images of the same distant quasar imaged around the center of the intevening galaxy. How does its symmetry compare to the two real presets we saw? Need a Hint?
			A)	This the the most perfect alignment possible, better than either of the presets.
			B)	This alignment is the least precise, with twice the vertical offset of the binary quasar, hence twice as many images.
			C)	This alignment can not happen in gravitational lensing.
			D)	The Cross has an alignment better than the binary image, but not as perfect as Einstein's Ring.

Here we explore how the gravity of dark matter, in whatever form, is detected by the lensing of light passing close to it from more distant compact collapsed star, just as Einstein predicted and was tested in the 1919 total solar eclipse.

5			Gravitational lensing is now being used close to home to map the dark matter in the Milky Way's halo. Nicknamed MACHOs (Massive Compact Halo Objects), they come in a variety of forms. How can gravity lenses help here? Need a Hint?
			A)	The dark matter can pass in front of a more distant star and eclipse the star entirely for weeks at a time.
			B)	The compact objects can transit the disk of a distant star and let us find its spectral lines during this transit.
			C)	The massive white dwarfs, neutron stars, and even black holes can lens the light of more distant stars and brighten them, with more mass creating greater lensing and brightening.
			D)	Such alignments are impossible; it takes the gravity of entire galaxies to make lensing work.