There was an interesting letter in this week's New Scientist about Dark Matter, that implies that it's existence is not open to question.
New Scientist has developed an annoying habit of declaring confidently that "Dark matter makes up 80 percent of the universe's matter ..." without qualification (for example, 2 May, p11). I was under the impression that dark matter is a hypothesis in search of confirmation - or did I miss the part where it was discovered and explained?
From Martin Savage, Jomtien, Thailand
The overwhelming view among physicists is that something unseen makes up 80 percent of the universe's matter, and that this "dark matter" refuses to interact with normal matter except via gravity. What exactly dark matter consists of is an open question, but not its existence.
The Editor, New Scientist
That's entirely not correct. The current best theories to explain what we see in the observable universe, require that to be a lot more gravity than the visible matter we can see would account for. To fill in this gap, physicists hypotheses that there must exist an additional source of gravity that can not observe by normal means. They term this additional source of gravity, dark matter. But to say that it's existence is not an open question is incorrect. It is the best theory that fits the facts at the moment, but it's existence is not proven ... yet. There are alternative theories.
Always keep an open mind.
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