I was landing coming in to land at Denver International on a flight from San Diego. I believe that we were somewhere over the Bailey, Park County area over the mountains. I looked out the window, positioned behind the right wing of the plane. A few hundred feet from the plane at a slightly lower altitude was a reflective chrome-like triangular craft with a noticeable dome of the same color in the middle of the top of the craft. It was approximately 30 feet long if I had to guess. I knew immediately it was not a plane because there were no visible seams, it was just smooth with the only change being the dome bump. I stared at it for a few seconds as it stayed almost perfectly aligned with us. I turned to wake my mother up who was in the middle seat next to me so that she could look, when we turned back to look it was still there, but my mother couldn't see well (cataracts). I turned away one more time, turned back and it was gone. I tried to spot it ahead of or behind us but it had vanished.