of course there are aliens, hell but are there alternate dimensions existing next to our own where other "aliens" exist maybe on our own earth. what could have visited earth in the past and may still be popping in from time to time are extradimensional beings living in a dimension close to our own. way some ufos are purported to move changing directions instantaneously phasing in and out changing size and shape etc could all be explained if something was intruding from another dimension.
lest that would be far more likely then aliens crossing 10s of thousands of years form the nearest stars to say hi or mess with our air traffic guys.
if the hubble deep field scan does not prove the existence of life out there, since even in the tiniest blackest empty space in the sky there are near countless galaxies with countless numbers of stars in them with planets orbiting a good number of those stars, it is near impossible that there is not life even intelligent life out there.