What started the birth of the universe? According to physics, the universe sprang into existence from a singularity — a term physicists use to describe regions of space that defy the laws of physics. They know very little about singularities, but it's known that such regions exist in the cores of black holes. It's a region where all the mass gobbled up by a black hole gets squeezed into a tiny point, infinitely massive, but also very, very small.

Imagine cramming Earth into something the size of a pinpoint. A singularity would be smaller.

The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields and high energy particles (e.g. cosmic rays).The Universe is incredibly huge. It would take a modern jet fighter more than a million years to reach the nearest star to the Sun. Travelling at the speed of light (300,000 km per second), it would take 100,000 years to cross our Milky Way galaxy alone. No one knows the exact size of the Universe, because we cannot see the edge – if there is one. All we do know is that the visible Universe is at least 93 billion light years across. (A light year is the distance light travels in one year – about 9 trillion km.)

Until about thirty years ago, astronomers thought that the universe was composed almost entirely of ordinary atoms. However, in the past few decades, there has been ever more evidence accumulating that suggests there is something in the universe that we can not see, perhaps some new form of matter.

As weird as it sounds, normal atoms almost certainly accounts for the smallest proportion of the Universe, somewhere between 1% and 10%.

In the currently popular model of the Universe, 70% is thought to be dark energy, 25% dark matter and 5% normal matter.