Our bewitching sky at night is blazing with fiery luminous objects, such as stars and galaxies. However, most of the matter in our Universe is "dark"--or, more precisely, invisible--and scientists are "in the dark" about its alluring and mysterious nature--hence its name, "dark matter"! This bizarre substance is believed to be composed of exotic non-atomic particles that do not interact with light--which is why it is invisible. Although scientists are in the process of rapidly closing in on the elusive identity of this weird and abundant stuff, its nature is still an intriguing puzzle. Adding to the puzzle, astronomers recently found that the merging galaxy cluster, Abell 520, which is 2.4 billion light-years away from Earth, possesses a core of dark matter and searing-hot gas that should be pulling in, with its mighty gravitational grip, many more galaxies than it apparently is--and this discovery is challenging existing theories suggesting that the dark matter should be anchoring these galaxies like a gigantic wad of glue, preventing them from wandering away into Space!
Luminous objects, such as stars and galaxies, make up only a small percentage of our Universe. We are composed of starry-stuff. The stars cooked the atomic elements that compose our bodies deep in their nuclear-fusing hearts, before they blasted themselves to pieces when they ran out of nuclear fuel, seeding the entire Cosmos with the ingredients to enable life to develop, on our own planet and elsewhere. The so-called "ordinary" atoms that make up such objects as stars, galaxies, planets, moons, and people, represent only a tiny fraction of the mass and energy content of our Cosmos (E = mc squared).
But the glittering stars are only the delightfully beautiful frosting on a magnificent Cosmic cake! The gigantic, starlit galaxies and unimaginably enormous clusters and superclusters of galaxies, are all embedded within halos of the mysterious dark matter. The dark matter, though never observed directly, is thought to exist because it exerts gravitational effects on objects that can be seen--such as stars and galaxies--even though it is invisible. In fact, the only way that the dark matter is known to interact with "ordinary" matter is through the force of gravity.
The true identity of dark matter is still unknown. The strange and abundant stuff weaves immense, web-like filaments throughout Spacetime, and does not interact with any form of radiation. The starlit galaxies are strung-out throughout this strange web-like structure like glittering beads on an intertwining Cosmic necklace.