@kennethamy,
"scientists believe they are there because they do observe their effects, and they postulate those entities as the best explanation of those effects..."
First, a quick apology for delay in reply, because circumstances have prevented...
I take your point, Kenneth, but I think the second half of your sentence expresses the real situation more accurately than the first half. The first half encapsulates the Baconian/Newtonian scientific attitude. When Newton published his theory of gravity, there were doubtless many natural-philosophers who said 'At last, now we REALLY know how the universe works!'
The second half expresses the more modern, post-Riemannian, post-Einsteinian attitude ('Let's assume that such-and-such is provisionally the case, without prejudice, because thus far that would explain the data we have...'). I think there is a difference between believing something is really there, and on the other hand, postulating an entity as a possible explanation of a phenomenon.
When a body of theoretical knowledge has developed sufficiently, one may assume with some justification that its original, basic terms describe something which may be loosely and coversationally termed as 'really existing'. For example, the theory of evolution by natural selection and the genetic theory of inheritance have both progressed so far that we can take it for granted that the word 'gene' corresponds to something which 'really exists'.