Dark Universe 101 | National Geographic

• [Instructor] The planets, moons, and stars make up less than 5% of all the mass in the universe. The rest lies in the realm of absolute darkness. The dark universe is the invisible, yet dominating, component of the cosmos.

It includes a substance called dark matter and a force called dark energy. While little is known about either, each helps us understand the universe beyond what the eye can see. Dark matter makes up about 25% of mass in the universe, but its composition is still unknown. The substance neither emits nor reflects light, and because of that, it cannot be seen.

The existence of dark matter can be inferred based on its gravitational pull on the visible universe. According to some scientists, this force is what prevents stars within galaxies from drifting away. Dark matter may have also played a role in the creation of the cosmos. After the Big Bang, dark matter formed a network of filaments across the universe and, in this network, the first stars and galaxies were born.

Not much is known about dark matter, but even less is known about dark energy. Dark energy is a mysterious force that fills the space between objects. It makes up about 70% of mass in the universe, plus it has a repulsive property that counteracts the force of gravity. The reason for dark energy's existence is unclear.

One theory states that it's the fifth force of nature. Yet another says it may help explain why the universe is expanding. In his Theory of General Relativity, Albert Einstein considered a repulsive force, similar to dark energy, that pushed space and particles apart. Called the cosmological constant, the variable may provide clues for the expansion of space and time.

Much about the dark universe is left to be discovered. But with every theory and technological development, secrets of the invisible cosmos will come closer to being revealed.