When looking at the theories for Dark matter from afar, it seems kind of humorous in a way when none of our instruments can detect it yet.(Current picture of Dark matter as shown on a black background) Sorry could not resist... Scientists are not 100% certain it exists. Dark matter is one of the few theories that is being used to explain the problem of unaccounted for mass in observed galaxies. Kindly note that mass is not weight in the way we describe it here on Earth. Mass is the quantity of matter an objects consists of. Weight is the effect gravity has on it. Since, by nature, dark matter is not detectable with conventional technology due to it's inability to emit radiation or light It is essentially invisible. To see Dark "Stuff" astronomers look at the behavior of objects near the dark matter.
One of the first to recognize something just wasn't adding up was Fritz Zwicky a Bulgarian born Swiss but worked in America astrophysicist. His "observations" of dark matter were done while studying the motions of faraway galaxies in 1933. Zwicky estimated the mass of the observed galaxies by measuring their brightness. He then used a different computation method to determine the mass. Hold the Phone! The result was 400 times larger than his initial method. Oddly enough Zwicky's research results were a lot like Dark matter. They were unnoticed until the 70's of the 20th century - losing decades of potential research in this field. Scientists realized that Zwicky's observations could explain some of their own. Today , dark matter is being taken seriously.
Some astronomers believe that over 20% of our universe is made up of what we call "dark matter", and another 67% of dark energy. Oh and the stuff we are familiar with and can actually look at only takes up 10% of the universe. Our universe is dominated by dark matter and dark energy, whatever they may be. The number might even be closer to 95% for Dark matter/energy.
Recently (2006) British researchers from the University of Cambridge made some interesting discoveries. Their calculations show that dark matter particles are moving at an amazing speed of 9 kilometers per second, a lot faster than previously thought. The researchers have also been able to determine the temperature of this particular dark matter, 10,000 °C.
Enough of all that... what is dark energy/ mass? The club seems to be getting crowded with MACHOs (MAssive Compact Halo Objects) and WIMPS ( Weakly Interacting Massive Particles) and Nonbaryonic particles This is mass but not Electrons Protons or Neutrons. Other candidates include Neutrinos and the evil twin,Massive Neutrinos, Axions, and supersymetric particles like Neutralinos . I do have to say that MACHOs have pretty much been voted out of the club since they do not come in large enough numbers to be a player but I love the acronym so I included it.WIMPS are making a case for the lead so far. Image Credit: Sky & Telescope / Gregg Dinderman
Right Now we are all over the map but Scientists have come to some conclusions. Dark Matter is most likely Nonbaryonic in nature, so generally the dark matter debate falls into one of three camps ( I will get to the alternate theories in a few ) Hot Dark Matter, Warm Dark Matter and oh yes, Cold Dark Matter. Each one right now has some of the questions answered but not all of them. Every day we are learning something more about the world we live in. I will lightly touch on alternate theories, well some of them.
The biggest line of reason to scrap the whole dark matter thing lies in our possibly incomplete understanding of gravity. Several system are being proposed but all have problems that cannot be answered yet.The MOdified Newtonian Dynamics (MOND) is one such theory. That theory has trouble reconciling the gravitational lensing events as Light is bent around galaxies from further out sources. Other systems trying to answer the problem of who hid the mass in the universe include The Tensor-Vector-Scalar which answers a bunch of the question of early models. Quantum mechanics has a candidate in there too and there is the Dark fluid answer. So far nothing is certain except we do not know yet. I suspect we will in the mid future. I look forward to that day when one team takes the prize and gets the Nike endorsements! All of the others start working on the next big thing... Until then ...
Clear skies and great seeing too