There is a line of thought that says that if something can't be observed, measured or defined scientifically then it doesn't exist. I think that way of thinking closes the mind.
Are you referring to anything or anyone specific here? The closest scientific principle I know to this is Occam's razor: do not multiply entities beyond that which is necessary. in other words, we cannot say that a thing does not or could not possibly exist, but we can say that to discuss such a thing without evidence is pointless unless you are discussing a hypothesis and how to test it. I'll believe in a god or gods when evidence gives me a reason to do so, and not before.
There is a lot we don't know or understand, so foreclosing the possibility of other states of being or consciousness is a mistake. We simply don't know, as you say.
Science and the scientific method have enabled us to understand a lot of the world around us. Its value is self-evident. But we shouldn't make the mistake of thinking that it is the only tool we have for gathering knowledge. It can't answer every question, and that's okay.
There certainly is much we don't know. (Job security for those in the sciences!) That does not, however, mean that it is reasonable to believe a proposition for which there is no reliable supporting evidence. It is a natural part of human nature to believe such things, but this is not the same as being reasonable. Science is demonstrably the most beneficial tool for understanding the world we inhabit, checking that knowledge against new information, and revising it when needed. I don't see religion as a tool for knowledge, rather as one used to cope with what is often a hostile existence, to find meaning where there is none given, and to provide certainty where little can be found. It is a tool I will avoid using. I won't look down on those who do, but it is perfectly justifiable to criticize the ideas that result from it.