Well Doctor, at the risk of sounding like a total fruitcake (PC? Don't care) I have an ongoing event in my life that somewhat subscribes to your theory. 5 months ago, out of the blue, I started getting daily visits every time I would lay down for a nap or to go to bed. Usually within a one to five minutes I will feel what I perceive to be an invisible cat walking on my bed. I've read about a lot of people having unseen cats walking on their bed but my visitor(s) go a couple steps further. They also manifest at other places though not so often, such as under my computer desk. These "cats" will lay against me in bed and purr. I can feel them. They are solid (though invisible) but if I attempt to touch them they disappear from the spot they are lying in. I've also had them lick my foot on many occasions. They appeared when I was living in Maryland and they "followed" me to Pennsylvania. I have had quite a few instances where I was nowhere near my bedroom (where bed and computer desk reside) and suddenly I would make a quick movement and I've bumped into them as though they were following me around. Not a thing has changed about my life other than this one bizarre occurrence.
I initially thought it was tactile hallucinations but after five months of this and it being strictly this one subject, I am starting to doubt that. I won't even be thinking of them (they're commonplace to me now) and I'll bump into something under my desk and there's nothing there to be seen. All the mannerisms are that of a cat with the exception that they're invisible. Also, apart from the purring they don't make any noise. Although I do hear them jump off the bed and hit the floor when I least suspect it. Well, I hope this doesn't lead to your busting a stitch laughing but I assure you that I am neither crazy nor delusional. For the sake of transparency I am almost 60 years old and I've been diagnosed bipolar. But that condition has been with me most of my life and I've never experienced anything remotely like this. So, in summary, if invisible aliens are taking the form of cats then I don't doubt your hypothesis/theory one bit.
Actually I don't think you're crazy - I've felt what seems like "invisible cats" walking on my back several times, even when I realized that the phenomenon was happening and I could actively observe what was going on. (And I'm a trained scientist). They may not be cats but some sort of unrecognized phenomenon, but who knows. After all that's the fun of science - detecting, experiencing and recognizing the unexpected.
As Shakespeare said “There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.”
- Hamlet (1.5.167-8), Hamlet to Horatio
Can't resist adding, I have had the distinct sensation of a cat jumping on to my bed near me. I have cats, so maybe that's why I thought it was a cat, but I was so certain of it that I turned on the lights to locate and expell the offending feline only to find nothing there. Subsequent instances are a little creepy since I'm aware it is not a cat or anything visible.
Based on the so-called scientific evidence of these invisible, mostly unheard and untouched aliens, it is likely that they are not aliens, but demons sent to do Satan's work. All the available evidence could be interpreted either way, but demons probably have more cause to be here than aliens. Aliens would likely want to make their presence known to us, whether they want to help us, or conquer us. Satan has much more reason to work secretly.