From Abracadabra to Zombies
In a nutshell: The Big Bang is the name given to the explosion that apparently happened 13.7 billion years ago and gave rise to our universe.
The current scientific model of our universe is that there are billions of galaxies, each with billions of stars and planets, and each moving away from Earth in all directions at a faster and faster pace. We live in an expanding universe where stars are still forming, joining other stars that have been here since shortly after the Big Bang. Yes, as you were reading this, the universe got a little bit bigger, stars exploded somewhere in the universe, and new stars were being formed.
The Big Bang is the name given to the explosion that apparently happened 13.7 billion years ago and gave rise to the stuff that came together to form our galaxies. Think of this explosion as if it were a sudden blast of air blowing up a balloon from the center of the balloon and the galaxies emerging in and on the surface of the "balloon" after millions of years of weird things happening. Scientists who study the Big Bang and try to figure out exactly what happened in the very beginning moments of our universe are called cosmologists. Kosmos is the Greek word for world or universe.
The galaxies that make up our universe differ in size and shape. Some are small and spherical. Others are large and shaped like spirals or whirlpools. Some galaxies have only a few million stars. Others have thousands of billions of stars. Galaxies are held together by gravity, one of the basic forces in our universe. In addition to stars and planets, galaxies have large chunks of matter (like asteroids), vast fields of gases and dust (much of it left over after stars explode), and a mystery substance called "dark matter."
Scientists calculate that there are about 80-100 billion galaxies in the observable universe and from 100-200 billion galaxies in the universe. (The universe is bigger than the observable universe. Much of our universe can't be observed ...yet.) In the past, the galaxies were closer together. In the beginning, they were one.
Our galaxy is called the Milky Way. Our Sun is just one of the 200-400 billion stars in the Milky Way. (The Greek word galaxias means "milky.") Our galaxy began forming about 13.2 billion years ago. The Sun formed about 4.6 billion years ago and is likely to exist for another 5 billion years before it becomes a red giant. The surface of the Sun is gradually becoming hotter. The Earth, which is about 4.5 billion years old, will be unable to sustain life in about 1 billion years because the heat from the Sun will have evaporated our atmosphere and our water. It is likely that the uninhabited Earth will be swallowed up by the expanding Sun when it becomes a red giant.
Scientists estimate that the Milky Way has at least ten times as much dark matter as it has stars and gas taken together. The existence of dark matter is known by its gravitational pull on normal matter. Scientists recently made a map of dark matter in a large section of space.
The human mind can't really grasp the hugeness of the universe or the immense length of time it, our solar system, and life on Earth have existed. Simple life forms, for example, first appeared on Earth at least 3.5 billion years ago. Humans have been around for only about 200,000 years. We are just one of many millions of species that have evolved on Earth.
Scientists estimate that 99.9% of all species that ever were are now extinct. The odds are that the human species will someday join the ranks of species that no longer exist. It is also a good bet that our extinction will happen long before Earth becomes uninhabitable. It is also a good bet that neither you nor I need worry much about either the end of Earth or the end of our species. It is very unlikely that while we are alive the Earth will suffer a catastrophic event like the planet being hit by a giant asteroid or anything of the sort. Rather than dwell on what will be in the very distant future, we might do better to remember that the Big Bang gave rise to subatomic particles, elements, space, time, and all the stardust that eventually found its way into your body and mine.
Currently, scientists do not know why the Big Bang happened or whether there are other universes besides our own. But even though our species has only been around for a very short time compared to how long the universe has been here, our generation has the chance to understand more about the universe and everything in it than any other creature that has ever existed on Earth. There has never been a more exciting time to live than now. We are also lucky in that we do exist, while uncountable gazillions of others will never get the chance to think, learn, explore, and discover new things about the only universe we know.
Origins of the Universe - National Geographic
How Many Galaxies in the Universe by Fraser Cain
NASA Astrophysics Image Gallery (the images above of a nebula and remnants of a supernova are taken from this site)