Over the last three decades, new technologies and ideas have driven cosmology forward at a rapidly increasing pace. Once a science of data-starved speculation, cosmology is now a full-blown race between theory and observations―the hallmark of a vigorous physical science. This race began in earnest in 1964, when scientists at Bell Laboratories, while attempting to understand radio antenna noise, discovered that some of that noise was a signal received from all directions in outer space. It was soon realized that this signal might be the cooled-down remnant of the radiation predicted by the Big Bang model. Cosmology finally had an observational foothold, a measurable remnant of the early universe, a probe to test the various cosmological models. The radiation was dubbed the cosmic microwave background radiation (CMBR). In 1990, early data from the Cosmic Background Explorer (LOBE, pronounced ho-bee) satellite showed that the CMBR had precisely the profile of intensity versus frequency to be consistent with the hot Big Bang model of the universe. In 1992, the COBE satellite produced another remarkable discovery. Data from a second experiment aboard the satellite showed slight variations of the CMBR intensity―with direction in the sky. The search for these variations had spanned 25 years. This discovery caused great excitement among cosmologists, because lumps in the CMBR are believed to be the ancestors of lumps of matter in our universe today. Making large surveys of the sky, astronomers are now able to locate the positions of thousands of galaxies in space and have found to their surprise that galaxies are far from uniformly distributed. Enormous sheets of galaxies enclose huge empty voids and form a structure that resembles a sponge or soap foam. Similarly, large-scale studies of galaxy motions show that huge regions of the universe are involved in high-speed bulk motion relative to the CMBR. The complexity and enormous scale of structure in the universe surprised cosmologists. The details of the statistical properties of the structure expected in the universe are dependent on the type and quantity of “dark matter” that dominates the universe. Cosmologists look forward to another exciting discovery―the identification of this mysterious dark matter. Over the decades, astronomers have gathered evidence of unseen mass binding together galaxies and clusters of galaxies, but the nature of the dark matter still remains a mystery. Is it something that we already know about, like the stuff that makes up our Earth and Sun? Or has nature concealed some completely new kind of matter from our earthbound physics experiments? Theoretical particle physics offers many exotic candidates, raising the possibility of not only discovering a major component of the universe but triggering a new era in particle physics as well. The search is on, in physics laboratories and at telescopes. A recent development is the search for dark matter by using its ability to bend light from distant stars or galaxies (gravitational lensing). Dark matter in clusters of galaxies and in the halo of our own galaxy is being studied with this elegant new technique.