What is Dark Matter?
This must be the first question that comes to mind when you see or hear of the term `dark matter’. Astronomers have been asking themselves the same question for years. The term conjures up images of something dark, energetic and mysterious. And it is.
Astronomers and cosmologists only have ideas of what it could be. Its existence and properties are inferred from its gravitational effect on galaxies, visible matter, radiation and the structure of the universe.
Dark matter is a term used in astronomy and cosmology to refer to a type of matter hypothesized to account for a large part of the total mass in the universe and exert significant force on ordinary matter. It makes up about 26.8% of the universe according to the Planck Mission Team.
Cosmologists study the universe trying to uncover its origin and predict its destiny. They know that there is something unknown and that has never been seen before making up a large percent of the universe. For lack of a better word, they have named this unknown shadowy mysterious thing dark matter.
What Do We Know About Dark Matter?
Scientists know more about what dark matter is not than what it is. It behaves different from normal matter that we are used to. It does not behave like the living and non-living things, planets, stars and galaxies. This ordinary matter is known as baryonic matter. Its fundamental unit is the atom.
Dark matter is not in the form of dark clouds of normal matter made up of particles known as baryons. If it were, we would be able to detect it.
Unlike baryonic matter, dark matter does not emit or absorb light and other forms of electromagnetic energy. That is why it is not visible directly even when viewed through a telescope.
Dark matter is not antimatter. The unique rays produced when antimatter comes into contact with matter and annihilates are not present.
It is not a large galaxy the size of black holes because a number of gravitational lenses show.
Possibilities of What Dark Matter Could Be
While most cosmologists agree that dark matter exists, they have more questions about it than answers starting from what it is. Is it exotic and undiscovered matter or just ordinary baryonic matter that is hard to observe or that has eluded detection up to now?
It is unlikely that it could be ordinary matter but MACHOs do exist. MACHOs stand for massive compact halo objects. They are large objects that live in the halos of galaxies but elude detection because they have such low luminosity. Examples of MACHOs are brown dwarfs, planetary materials, dim white dwarfs and black holes. However, there is much more dark matter than could be accounted for by MACHOs alone.
Astronomers think it is more likely made up of entirely new type of matter built from a new kind of elementary particles. This is the most popular view. It states that dark matter is made up of exotic particles such as axions and Weakly Interacting Massive Particles (WIMPs).
How Dark Matter was Discovered
The concept of dark matter was first postulated by Jan Hend Rik Oort in 1932. He noticed that the stars in our galactic neighborhood were moving more rapidly than calculations predicted. For them to move faster than was expected, there had to be extra mass responsible for the increase in velocity He used the term dark matter to describe the unidentified mass.
One year later in 1933, Fritz Zwicky started studying galaxies in the Coma Cluster. He used luminosity measurements to calculate the mass that should have been in the cluster and used the mass to calculate its most likely velocity. Then he measured how fast the Coma Cluster was actually moving and found that the actual speed was faster than he had calculated and expected.
He explained this difference by suggesting that more mass was hidden among the visible matter and was responsible for the higher velocity. He called the invisible mass dark matter.
Observations of clusters of galaxies show that the motion of stars within a galaxy suggests that they are bound by a total gravitational force due to about 5-10 times as much as can be accounted for by luminous matter in the galaxies.
In 1950s and 60s, astronomers started measuring the rotations of spiral galaxies to determine how fast they rotated. It is then that they made a discovery.
Near the center of our galaxy, The Milky Way, there is more visible matter so astronomers expected stars there to move faster than stars at the edge of the galaxies where visible matter is less. That was not what they observed. Stars at the edge of our galaxy had the same rotational velocity as stars near the galactic center.
In the 1970s, Vera Rubin confirmed this applied to other galaxies as well and not just the Milky Way when he studied rotational speeds of galaxies.
The most likely explanation put forward by all these scientists was that galaxies and galactic clusters contained an invisible form of matter causing the gravitational effects. This is dark matter. They then focused their attention to studying and collecting more evidence to support the existence of dark matter.
They studied clusters of galaxies bound together by gravity trying to find the mysterious composition of galaxies. They used X-ray telescopes such as the Chandra X-ray Observatory to study the clusters.
Evidence That Dark Matter Exists
Dark matter is invisible and cannot be detected. So how do we know that it exists? There are plenty of reasons to believe the universe is composed of dark matter that influences the evolution of the universe gravitationally but is not visible directly in our present universe.
Its existence was hypothesized after discrepancies were noticed between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from their luminous matter which contains stars, dust, gas and other visible matter.
Observations of motions of astronomical objects, stellar galactic, galaxy clusters and superclusters show that they have more mass than can be attributed to luminous stuff.
Dark matter could be hot or cold. Cold dark matter travels at slow speeds or have little pressure while hot dark matter moves rapidly.
Where Is Dark Matter in The Universe?
Computer mapping models show that dark matter could be everywhere binding the universe together and acting like an invisible connective tissue.
In January 2012, an international team of researchers made the largest map of the invisible stuff in the universe ever made. This map portrayed dark matter as a vast web of dark matter stretching across space and mixing with normal matter. They used data collected by the 340-mega pixel camera on the Canada France-Hawaii Telescope (CFHT) on Mauna Kea Mountain in Hawaii.
Explorations of the universe to find out more about dark matter continue.