We were driving through Colorado on our way from LA to Chicago. It was getting into the night- 10-12, and I noticed something in the night sky as we were driving down the highway. My girlfriend saw it too, but was clearly not interested in it as I was. It was large, but at first paid no special mind to it. A while later it was still there, but it was clearly not a plane, and it continued to follow the highway. I rolled down the window but heard nothing special. It was lit by a few lights. After some time, I told my gf, that we needed to pull off the highway to take a look, because I had to know what it was. We did. The thing flew over us, it was large and silently blotted out the stars above us. We guessed it had to be a large blimp or something, but it doesn't make sense. I think we would have heard a blimp. We eventually got back in the truck and got going. The thing was gone. I am familiar with aircraft- it was not a plane, it was moving too slowly. It may have been a blimp at night, but did not make a sound and the lights seemed wrong. I don't know if it was something alien, it may have been a large secret air ship, but it was there, and huge. At least the size of large aircraft.