The general assumption seems to be that spaceships and or UFOs operate in our realm of 3 physical dimensions plus time.  In my opinion, that is an extremely weak assumption, simply hubris.

In fact we know absolutely nothing about 90% of the matter and energy in the universe.  We don’t even know what we don’t know.  We only know that we cannot see or touch or sense about 90% of the stuff that surrounds us in the universe because it does not interact with light.  And that is about the only thing we know about it.  That stuff is called dark matter and dark energy and we can sort of estimate its mass by subtracting an estimate of the mass of the stars, planets, gases, etc in the universe that we can see and sense, that is, the stuff that does interact with light.  But, that estimate could be completely wrong.  We don’t even know enough about dark energy and dark matter to estimate the uncertainty in our estimate.

The volume around every molecule in the air around us is 99% empty space.  The volume around every nucleus in every atom inside every molecule in the air around us is 99% empty space.  Given what we do know about the difficulties of travel in 3D space and time, in other words in our known realm, in my opinion it is 99% more likely that aliens are standing here right beside us now, just 1 molecule’s distance apart from us; that seems much more likely than aliens traveling great distances in space ships and such to explore our planet.

The greatest body of knowledge in physics today, and the work of virtually all senior physicists in every university is known as string theory.  Today, according to Smolin in the reference below, probably no physicist becomes a tenured professor unless they believe and publish on string theory.   However, none of them have even the tiniest bit of empirical, real world evidence to support string theory.  It is entirely theoretical with no hard evidence.  It is all math.  That math says there are multiple dimensions in the universe and that everything – everything – is connected.  Insofar as I understand this theory, all matter – even the farthest points of the universe that we can sense or probe with light – is connected in space and real time by something we can imagine as strings.

References:

(1) The Trouble with Physics, The Rise of String Theory, The Fall of a Science, and What Comes Next. by Lee Smolin.  https://www.amazon.com/Trouble-Physics-String-Theory-Science/dp/061891868X?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-ffab-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=061891868X

(2) https://www.popularmechanics.com/military/research/a29771548/navy-ufo-witnesses-tell-truth/