Yes of course, this can be beings invisible the eye. But we feel them, in ourselves and near. About aliens, we think that they should be like movies or like any other humanoid creatures. Who said that? They may not be quite like anyone else either, to be incorporeal and feed on other energies. After all, we can only see our range of perception ( as purity on the radio ). But in the whole universe there are other frequencies that we can not even understand, realize. And this proves that why not live in these spectra of other creatures.