The Big Bang Theory
The Big Bang theory is the most widely accepted scientific explanation of how the universe
formed. To understand this theory, start by picturing the universe expanding steadily. Then,
reverse the direction of time, like pressing the “rewind” button on a video player. Now the
universe is contracting, getting smaller and smaller. If you go far enough back in time, you
will reach a point when the universe was squeezed into a very small volume.
According to the Big Bang theory, the universe began about 13.7 billion years ago, when
everything in the universe was squeezed into a very small volume, as described above. There
was an enormous explosion—a big bang—which caused the universe to start expanding
rapidly. All the matter and energy in the universe—and even space itself—came out of this
explosion.
After the Big Bang
In the first few moments after the Big Bang, the universe was extremely hot and dense. As
the universe expanded, it became less dense and it cooled. After only a few seconds, the
universe had cooled enough that protons, neutrons, and electrons could form. After a few
minutes, hydrogen could form and the energy in the universe was great enough to allow
for nuclear fusion, creating helium atoms in the same way we learned that a star can make
helium out of hydrogen atoms, even though there were no stars at this point in the universe’s
history. The first neutral atoms with neutrons, protons, and electrons, did not form until
about 380,000 years after the big bang.
The matter in the early universe was not smoothly distributed across space. Some parts of
theuniverseweremoredensethanothers. Theseclumpsofmatterwereheldclosetogetherby
gravity. Eventually, these clumps became the gas clouds, stars, galaxies, and other structures
that we see in the universe today.
Dark Matter and Dark Energy
The Big Bang theory is still the best scientific model we have for explaining the formation of
the universe. However, recent discoveries in astronomy have shaken up our understanding of
the universe. Astronomers and other scientists are now wrestling with some big unanswered
questions about what the universe is made of and why it is expanding like it is.
Dark Matter
Most of the things we see out in space are objects that emit light, such as stars or glowing
gases. When we see other galaxies, we are seeing the glowing stars or gases in that galaxy.