I think one of the videos I watched recently mentioned something along the lines of that if aliens were to come to earth they would have to adapt their form to fit something sustainable on earth. While they could be made up of something we might not necessarily consider material, if they wanted to be made visable to humans they would need to have a form similar to a human or animal. Well the video specifically said they would have two legs and 2 arms and more than likely stand upright and be tan but that just sounds like absolute bullshit to me because there are so many other animals on eart that do not fit that picture, ones that hop, slither, whatever, so while I think that the aliens would have to have adapted at some point to the extent where they could travel a significant distance through space, I do not necessarily believe that video was totally accurate in that respect. But, there could be aliens all around us in various different forms that we just do not see, some so microscopic, invisible, or composed of something that we cannot percieve. Who is to say that the earth is not composed of other senses that we as humans cannot percieve, but that aliens can and take advantage of?