Only five percent of the universe is visible. What is the rest made up of?
Dark Matter and Dark Energy
The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary matter of stars down to the molecules of DNA makes up less than 5 percent of the mass of the universe.
The rest of the universe appears to be made of a mysterious, invisible substance (something) called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent).
Scientists have not yet observed dark matter directly. It doesn't interact with baryonic matter and it's completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments. But scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.
Scientists have a few ideas for what dark matter might be. One leading hypothesis is that dark matter consists of exotic particles that don't interact with normal matter or light but that still exert a gravitational pull. Several scientific groups, including one at CERN's Large Hadron Collider, are currently working to generate dark matter particles for study in the lab.
Dark energy is even more mysterious, and its discovery in the 1990s was a complete shock to scientists. Previously, physicists had assumed that the attractive force of gravity would slow down the expansion of the universe over time. But when two independent teams tried to measure the rate of deceleration, they found that the expansion was actually speeding up. One scientist likened the finding to throwing a set of keys up in the air expecting them to fall back down-only to see them fly straight up toward the ceiling.
Scientists now think that the accelerated expansion of the universe is driven by a kind of repulsive force generated by quantum fluctuations in otherwise "empty" space. What's more, the force seems to be growing stronger as the universe expands. For lack of a better name, scientists call this mysterious force dark energy.
Now that we see the expansion of the universe is accelerating, adding in dark energy as a cosmological constant could neatly explain how space-time is being stretched apart. But that explanation still leaves scientists clueless as to why the strange force exists in the first place.
The following short video will help explain matters better (no pun intended).