There’s little question that one of the strangest rabbit holes in cosmology today is Dark Matter. Despite years of study, we still know next to nothing about this form of matter that apparently makes up about 85 per cent of the matter in the universe.
For galaxies to rotate as they do, dark matter seems to have to be present. Though there are competing hypotheses that involve problems with our understanding of gravity itself rather than any form of dark matter.
The idea is simple on its face. For galaxies to behave as they do, they need more gravity. More than the sum of the visible materials within them can provide. Therefore, there must be other matter that we can’t see, a lot of it. And for that to be the case, it would need to be in the form of some unknown kind of matter. A matter that cannot be detected through normal means. This could mean that dark matter is some kind of weakly interacting subatomic particle. It is very abundant in the universe but just doesn’t interact with anything else except through the force of gravity.
But since dark matter is linked with the behaviour of galaxies, is it possible to have a galaxy made up entirely, or nearly so, of only dark matter? Such a thing would present a large amount of gravity. But very few stars and would be almost, or possibly wholly invisible. Think a gravity source of high power that you have no way to detect other than it’s gravitational effects on different objects.
But what might make them visible is if they contain a lot of hydrogen gas. Hydrogen has a new property with essential implications. It emits radiation. Known as the 21 centimetres or hydrogen line, neutral hydrogen emits radiation at the frequency of 1420 Mhz. Frequency is SETI’s primary target frequency. Due to the notion that since hydrogen is the most common element in the universe. An alien civilization doing science would know of the 1420 Mhz hydrogen line. They would assume everyone else doing science would know as well, and use that as a natural basic broadcast frequency for contact signals.
It is nature’s way of providing a natural place for alien civilizations to discover each other. And know where to tune the radio dial. And, indeed, the infamous and still unexplained Wow! Signal was at 1420 Mhz. And no, that signal was not due to comets, a common misconception these days. Despite the media attention on that hypothesis, and the idea itself didn’t get much traction in the scientific community since we’d see these kinds of emissions with other comets, and that hasn’t been the case.
Alien signals aside, regular natural neutral hydrogen does emit this radiation so we can see it. So if large amounts of atomic hydrogen are spotted in space, but with no optically visible counterpart normal galaxy, then it’s a candidate for a dark galaxy. Or it’s a gas cloud. But the most significant indicator would be the gravitational effects of the cloud on objects around it. If the behaviour of those objects indicates a reliable gravity source, then you might well have a dark galaxy. And, there are candidates.
The galaxy NGC 4254 is being pulled on by some authoritative gravitational source, which appears to be a cloud of hydrogen gas called VirgoHI21 that shouldn’t produce the gravity needed to do that on its own. But an underlying dark matter galaxy could. But what could these dark galaxies mean? One option is that they are newly forming galaxies, where the stars forming within them have yet to ignite. That would imply that the Milky Way might once have been a dark galaxy.
Indeed, thinking on this is that massive galaxies like the Milky Way formed through the merger of smaller galaxies, and possibly also dark galaxies. If dark galaxies are bringing lots of hydrogen with them, then that would provide the raw materials for the formation of large amounts of stars leading to the large galaxy we have today. In addition to the concept of dark galaxies, more mysteries are surrounding dark matter.
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