The Universe is thought to have been born about 13.8 billion years ago in the form of an exquisitely small searing-hot broth composed of densely packed particles–generally simply referred to as “the fireball.” Spacetime has been growing colder and colder ever since, as it expands–and accelerates as it expands–from its original furiously hot and glaringly brilliant initial state. But what composes our Cosmos, and has its mysterious composition changed over time? Most of our Universe is “missing”, meaning that it is made up of an unidentified substance that is called dark energy. The identity of the dark energy is probably more mysterious than that of the dark matter. Dark energy is causing the Universe to speed up in its relentless expansion, and it is often thought to be a property of Space itself.
On the largest scales, the entire Cosmos appears to be the same wherever we look. Spacetime itself displays a bubbly, foamy appearance, with enormous heavy filaments braiding around one another in a tangled web appropriately referred to as the Cosmic Web. This enormous, invisible structure glares with glowing hot gas, and it sparkles with the starlight of myriad galaxies that are strung out along the transparent filaments of the Web, outlining with their brilliant stellar fires that which we would otherwise not be able to see. The flames of a “million billion trillion stars” blaze like dewdrops on fire, as they cling to a web woven by a gigantic, hidden spider. Mother Nature has hidden her many secrets very well.
Vast, almost empty, and very black cavernous Voids interrupt this mysterious pattern that has been woven by the twisted filaments of the invisible Web. The immense Voids host very few galactic inhabitants, and this is the reason why they appear to be empty–or almost empty. The massive starlit dark matter filaments of the Cosmic Web braid themselves around these black regions, weaving what appears to us as a twisted knot.
We cannot observe most of the Universe. The galaxies, galactic clusters, and galactic superclusters are gravitationally trapped within invisible halos composed of the transparent dark matter. This mysterious and invisible pattern, woven into a web-like structure, exists throughout Spacetime. Cosmologists are almost certain that the ghostly dark matter really exists in nature because of its gravitational influence on objects that can be directly observed–such as the way galaxies rotate. Although we cannot see the dark matter because it doesn’t dance with light, it does interact with visible matter by way of the force of gravity.
Recent measurements indicate that the Cosmos is about 70% dark energy and 25% dark matter. A very small percentage of the Universe is composed of so-called “ordinary” atomic matter–the material that we are most familiar with, and of which we are made. The extraordinary “ordinary” atomic matter accounts for a mere 5% of the Universe, but this runt of the cosmic litter nonetheless has formed stars, planets, moons, birds, trees, flowers, cats and people. The stars cooked up all of the atomic elements heavier than helium in their searing-hot hearts, fusing ever heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). The oxygen you breathe, the carbon that is the basis of life on Earth, the calcium in your bones, the iron in your blood, are all the result of the process of nuclear-fusion that occurred deep within the cores of the Universe’s vast multitude of stars. When the stars “died”, after having used up their necessary supply of nuclear-fusing fuel, they sent these newly-forged atomic elements singing out into the space between stars. Atomic matter is the precious stuff that enabled life to emerge and evolve in the Universe.
The Universe may be weirder than we are capable of imagining it to be. Modern scientific cosmology began when Albert Einstein, during the first decades of the 20th-century, devised his two theories of Relativity–Special (1905) and General (1915)–to explain the universal mystery. At the time, astronomers thought that our barred-spiral, starlit Milky Way Galaxy was the entire Universe–and that the Universe was both unchanging and eternal. We now know that our Galaxy is merely one of billions of others in the visible Universe, and that the Universe does indeed change as Time passes. The Arrow of Time travels in the direction of the expansion of the Cosmos.