It is possible that a supposed discussion of "dark matter and dark energy" and others could have been as you say, banished.
But I have seen no actual evidence of this sort of thing happening on this board.

What does get objected to, is the manner in which such things might be discussed.

This is a fact: There is no credible evidence of UFOs [meaning aliens in spacecrafts] or ghosts.

To deal with such issues as UFOs or ghost as though there was valid evidence is fraudulent. Pseudoscience.

And no, it's not a responsibly for rational people to prove that something is fraudulent. If you want to play the rules of science, the burden is on the one who is making "the claim".

I have discussed matters related to aliens on Space board. SETI research is actually a fairly common topic on the Space board. So, it's not an issue of people being intolerant of the possibility of the existence of aliens.

The problem has to do with the apparent non comprehension of some to understand rules used in science.

For instance, there is a reason why one uses such a thing as the double blind experiment:

"The blind method is a part of the scientific method, used to prevent research outcomes from being influenced by either the placebo effect or the observer bias. To blind a person involved in research (whether a researcher, subject, funder, or other person) is to prevent them from knowing certain information about the process. The terms blind (adj) or to blind (vt) when used in this sense are figurative extensions of the literal idea of blindfolding someone. Blinded research is an important tool in many fields of research, from medicine, to psychology and the social sciences, to forensics."
http://en.wikipedia.org/wiki/Double-blind

In other words, honest people can "see something" that does not exist.
I use the word "honest" in the sense that the person can pass a lie detector test- they are not aware of being deluded. Bias [which it is common or "normal"] can override perception.

Science basically is system in which it's purpose is to defeat bias. Not that it's always successful- it's more of an improvement in the chance of being successful.
The more the rules of science are applied the higher the chances- and part of these rules include that any result should be able to be repeated by anyone who repeats the experiment.

So, if ghosts or UFO can only appear to a "selected few" as "the way it's ordained"- this by itself, removes it from science. It might be considered an art, but it's not science.
Science can't prove what is not. Science attempts to prove what is.