“They do not emit electromagnetic waves. They do not interact with light. They have never been directly observed”Louise Flanagan unravels the history behind one of physics’ least understood concepts: dark matter and dark energy.[br]JUST because you can’t see it doesn’t mean it’s not there – this may sound like something from a horror film trailer but this statement also holds true for physics’ most puzzling and mysterious invisible entities: dark energy and dark matter.Together, dark energy and dark matter make up 95% of the universe. Only 5% is made up of “normal matter”, all the stuff we can see (atoms, stars, trees, and humans). Dark energy (68%) is thought to be a repulsive force responsible for the accelerating expansion of the universe, while dark matter (27%) is believed to be a heavy particle whose strong gravitational pull allows galaxies to exist without being torn apart.But we can’t see either of them. They do not emit electromagnetic waves. They do not interact with light. They have never been directly observed. If these mysterious dark entities are so invisible, how do we know they are there at all?Our awareness of dark energy began in the twentieth century, a time of dramatic developments in astronomy. The universe, which was once thought to be static and consist of just the Milky Way, was now proven to contain billions of stars and galaxies, and shown to be expanding.It was thought that this expansion would eventually slow down because of the effects of gravity pulling matter together, but this idea was turned firmly on its head in 1998 when two independent research groups showed that the rate of expansion of the universe was not slowing down at all. It was accelerating.There had to be some kind of explanation for this. Step forward: dark energy – a term that was given to the “something” that was causing this acceleration. It appeared that dark energy must be some kind of repulsive force that was evenly distributed through space, and whose properties did not get diluted when it expanded, pushing galaxies further and further apart over time.While there have been many suggestions, we don’t yet know what kind of “thing” dark energy might be. Like dark energy, the notion of dark matter came about to fill a gap in observations made in space. The amount of “normal”, visible matter in space did not have enough mass by itself to exert a gravitational force strong enough to build galaxies and other complex structures. If only the visible matter existed, then the stars that make up galaxies would likely exist in isolation, scattered all over the universe.Something had to be providing this attractive gravitational force to hold the galaxies together, something dense: dark matter. Though invisible, and since dark matter is massive, its gravitational effects are strong.Concentrated regions of dark matter can actually be detected because gravity interacts with light from distant galaxies and bends it, known as gravitational lensing. This bending of light in space has been captured in photos and has enabled the creation of dark matter maps. It appears that unlike dark energy, dark matter is unevenly distributed and is more like an interconnected web that extends across the universe, holding visible matter in its invisible threads. Ongoing attempts have been made to detect candidate dark matter particles directly but so far none have been successful.There is an awful lot yet to be learned about dark energy and dark matter but it seems that our eventual understanding of them will give us an insight into how the universe has been structured from its origins to now.