Thank you again for all the answers. And especially Strange, for trying to answer the original question. I would have counter statements, but shall think it through later ones more.
To get back to the topic and the question on how to scientifically ask the original question.
With the given information on this thread, let me try to take a shot at it… Would the following way be a more scientific way of asking the same thing (by giving an example)?:
“(If it turns out there is an increasing amount of dark stuff in the observable universe)
Let us consider the observable universe as a filled water balloon and dark stuff (-matter, -energy or something) as air. And also, that someone has sticked a straw in the balloon, with the other end of the straw in the middle of the balloon and the other end outside the balloon.
Now, if there is an increasing air bubble inside the water balloon. Couldn't we, by studying the border of the air bubble and the water, determine if someone is blowing in to the straw to make the increasing air bubble or if someone is stretching the water balloon to get the air sucked in? Maybe by, for example, if there was a bigger or a smaller vacuum cap at the border than in elsewhere in the bubble/balloon”
This question/hypothesis might still be bad (and as untrue as the original), but was this any better for a question/hyphothesis for discussion? This should, at least in my theory, have a little bit better base for the mentioned requirements; 1. mathematical model 2. testable predictions 3. experiments/observations to the those predictions.