Amongst, the above energy spectrum what the naked human eye can see, is represented as under -
For the other regions of the spectrum, left and right of the "Visible" region we need to rely on many devices (complex and simple). But have we ever paused and thought that are we really sure that the spectrum that we know contains all possible "Energy Forms" available in the vast span of the universe?
What if there are other forms of energy beyond Radio and Gamma Rays. None of the equipments available today would be able identify those Energy. They would be BLACK to us. Till date all the images relayed to us by Hubble are nothing but graphic representation of different energy patterns put together, which are comming from a common source. What if a celestial body emmit other patterns of energy. Hubble won't be able to understand that energy because we don't understand that energy and the only image that Hubble would be able to draw would be something that would make little or no sense (again this is only my guess and I'm not questioning the Genius behind Hubble.). Again, another question that spontaneously comes to my mind that what if we could camafauldge the energy being radiated from our planet ..... i.e. what if we could create a force field arround out planet that would stop of any of the energy radiations from getting transmitted beyond Earth's gravitational field? Wonder what would Hubble see in such a situation.
Therefore what I think is, if we think there are Extra Terrestrial Intelligent Life forms other than us (which we know there are!!) and they do visit us (which we know they do!!) then they should have the technology to do what I've stated above, because it's a real smart move to keep the element of surprise going because that's the best way for defending oneself and one's technology. It's an inter planetary Hide and Seek, magnitude we can't even imagine where an entire Planet and it's civilization is being hidden from those spying eyes of technology!!!