The US defence agency has admitted to a dossier on so far unidentified flying objects for which they can offer no explanation. Many, around the world, see this as vindication for widely held belief that such craft had been observed and were likely manned by 'aliens'.
Almost all the theories propose that these 'aliens come from other solar systems but I suggest that the answer may be closer to home and that they are not necessarily masters of space travel but of trans-dimensional travel..
To understand this it is necessary to imagine that spacetime as we are aware of it, where all aspects are positive, has two other aspects, a negative and imaginary. The Davies Hypothesis, see: https://www.amazon.com/gp/product/B00HU1H7SA/ref=dbs_a_def_rwt_bibl_vppi_i0
explains the basis for this new reality but here I shall just assume the existence of another side to our universe where dimensions adopt a negative value. This is not the negative as in a one dimensional line showing positive at one end; this universe must be considered as acting at right angles to the universe we exist in. This means that normally we cannot trespass into our sister universe without the knowledge to travel between dimensions.
What does this universe look like? Everything would probably be strange but the important thing is that gravity would still forge large scale structures because all matter would be attractive because the force of gravity is positive.
Normally G ~ f (m1*m2)/r^2. In our sister universe: G ~ f(-m1 * -m2)/r^2.
In other words, suns would be formed and their attendant planets, which may be able to support life.
Could it be that aliens have been crossing the dimensional divide and entering our side of our dual universe without having to cover vast distances at near the speed of light. To do this their technology would necessarily be more advanced than ours.