In five short years, skeptics have gone from doubting that microlensing could ever be detected conclusively to using it as a tool study our Galaxy and its contents (Gould 1996). One of the convincing pieces of evidence that microlensing has been detected is that the brightness of almost all microlensed stars reported by the detection teams have not varied again in several years of subsequent monitoring. This is important, since many, many "variable stars" are known to change in brightness rhythmically, not because they are focused by an unseen microlens, but because their atmospheres pulsate (changing the effective brightness and color of the star) or because they are periodically eclipsed by a partner star locked into an orbit around them. Microlensing is so rare, however, that a change in brightness due to lensing in a particular star should occur no more than about once per million years. In addition to finding the one-in-a-million star that is microlensed, astronomers must also monitor the 99% of stars that do not vary in brightness and the 1% that vary due to pulsation or eclipsing binary stars. Since millions of stars are being monitored in total, tens of thousands of variable stars have been discovered by these teams. Most of the variable stars known in our Galaxy and the Magellanic Clouds were discovered by astronomers looking for microlensing, revolutionizing the field of variable star research.
Although variable star research is a crucial field of astronomy, it is a ``by-product'' of the microlensing surveys. What has been learned about the primary topic motivating these surveys, namely the mass content of the Milky Way? To date, the number of microlensed stars discovered in the LMC (a few tens) and Galactic Bulge (a couple hundred) and the duration and amplitude of their light curves appears to differ from the initial predictions. More microlensing seems to be in progress toward the center of the Milky Way and less in the direction of the Magellanic Clouds than would be expected if dark matter alone was responsible.
The nature and distribution of these microlenses is not yet completely understood. Some astronomers argue that a fair fraction of the dark matter of the Galaxy must be composed of compact objects capable of lensing. Others are not convinced, arguing that the lensing from ordinary stars has been underestimated. Many astronomers feel that a large fraction of the lensing toward the center of the Galaxy is caused by stars in an elongated region of high stellar density called the "Galactic bar." Most astronomers agree that the center of the Galaxy contains more total mass than first estimated, but disagreement still remains as to whether some of this lensing mass is composed only of normal stars or whether a significant fraction of the lensing is caused by so-called "brown dwarfs,'' low-mass failed stars. Nearly everyone agrees that it is still too soon to draw firm conclusions, and so the experiments continue to gather the data necessary to settle the issue.
Since there is no reason to believe that our Galaxy is very different from any other, if microlensing can help us understand what sort of objects are responsible for most of the mass of the disk and dark halo of our own Galaxy, we will probably have learned about the content of most other galaxies as well.