Mysteries of the universe that still puzzle scientists

The Big Bang, the most widely-accepted theory for the origin of the Universe, took place 13.7 billion years ago. In one instant the Cosmos were created, and with this countless unsolved mysteries that have puzzled scientists for centuries. The questions are as intriguing as they are baffling: what's inside a black hole? Why does space roar? Are we alone?

Click through and discover the mysteries of the Universe.

What is a light-year?

Before we go any further, a word about light-years. For most space objects, light-years are used to describe their distance. A light-year is the distance light travels in one Earth year. And get this: one light-year is about nine trillion km (six trillion miles).

Dark matter

Incredibly, the numerous planets, stars, galaxies, and everything else we can see make up less than 5% of the total Universe. The odd thing is that 26.8 % is a substance scientists label as "dark matter." It doesn't interact with light or visible matter, but can be detected through its gravitational influence on the movements and appearances of other objects in the Universe, for example stars or galaxies.

Dark energy

And what of the other 68% of the Universe? Cosmologists call this "dark energy," the mystery force that rules the Universe. This phenomenon overwhelmed gravity and gained control of the Universe about five billion years ago. It is invisible, fills all of space, and its repulsive gravity is speeding up the expansion of the Cosmos.

Milky Way

Our own galaxy, the Milky Way, is a barred spiral-shaped galaxy and includes our own solar system. The oldest stars in the Milky Way are nearly as ancient as the Universe itself, probably formed after the Big Bang.

Stellar jet

Stellar jets are beams of ironized matter ejected from some astronomical objects, but usually produced by young stars still in the process of forming.

What caused the Big Bang?

Around 13.7 billion years ago, everything in the entire Universe was condensed in an infinitesimally small singularity, a point of infinite denseness and heat. Suddenly, from this mere dot, a huge explosion expanded space like a balloon. From a single atom, the Universe grew to bigger than a galaxy. It is still expanding today.

Supernova

Supernovae are violent stellar explosions that litter the Cosmos. One of the brightest, named by scientists as LMC N49 (pictured), is approximately 160,000 light-years from Earth.

Thousands of galaxies

And there are thousands of galaxies out there. Even the small dots seen in this image are entire galaxies, and are about 13 billion light-years away.

Black hole

A black hole is a place in space where gravity is so strong that even light can't get out. This is because matter has been squeezed into a hugely compressed area. This happens, for example, when a star is dying. Scientists reckon there are millions of these cosmic bodies in our galaxy, but no one knows what they contain. This image is of a black hole in Messier 87, a supergiant elliptical galaxy in the constellation Virgo.

Exoplanets

An exoplanet is a planet beyond our solar system. Most exoplanets orbit stars, but free-floating exoplanets, called rogue planets, orbit the galactic center in permanent darkness and are untethered to any star. Kepler-186f (pictured as an artist's concept), is the first exoplanet discovered in what scientists call the habitable zone—the region around the host star where the temperature is right for liquid water.

Are we alone?

In this artist's impression, the planetary system around the red dwarf, Gliese 581, is pictured showing what astronomers believe is another as yet unnamed Earth-like planet found outside our solar system, a planet that could have water running on its surface. And it's a lot nearer to us, a mere 20.5 light-years away, in the constellation Libra. Suddenly, the potential for extraterrestrial life seems a lot more real.

Titan

Titan is the largest moon of Saturn and the second-largest natural satellite in the solar system. Titan is the only moon wrapped in a thick atmosphere and has a surface of rock-hard water ice. Intriguingly, it also likely has a liquid water ocean beneath its surface. Image: NASA/JPL-Caltech/Space Science Institute

Cosmic rays

Cosmic rays are high-energy protons and atomic nuclei that zap through space at nearly the speed of light. They most likely originate from the Sun, from outside of the solar system in our own galaxy, and from distant galaxies. This ionizing radiation is harmless to us because it bounces off the Earth's atmosphere. However, cosmic rays have been blamed for electronic circuit problems and data crashes in satellites and other machinery. Image: NASA

Callisto

Callisto is possibly the most highly cratered body in the solar system. It's the second-largest moon of Jupiter, after Ganymede, and thought to be a long dead world, with hardly any geologic activity on its surface. It's one of the oldest landscapes in the Cosmos, dated back to at least four billion years. Image: NASA

Fermi Bubble

Discovered by cosmologists as recently as 2010, Fermi Bubbles are massive, mysterious structures that emanate from the Milky Way center and extend roughly 20,000 light-years above and below the galactic plane. They emit high-energy gamma rays and X-rays, invisible to the naked eye.

Pulsar

A pulsar is a highly-magnetized rotating neutron star so named because it emits beams of electromagnetic radiation out of its magnetic poles at regular intervals. But no one is quite sure why. Even more puzzling is that occasionally these stars stop pulsing.

Atlas

Saturn's flying saucer moon, Atlas is another solar system oddball. An inner satellite discovered in 1980, Atlas appears like a deep space UFO. Image: NASA/Jet Propulsion Lab-Caltech/Space Science Institute

Phobos

Phobos, the inner and larger of Mars' two moons (the other being Deimos), is thought by scientists to be in a "death spiral" in that it's slowly orbiting toward the surface of Mars. The natural satellite is named after the Greek god Phobos, the personification of fear and panic, and the origin of the world phobia. Image: NASA/JPL-Caltech/University of Arizona