It was sometime in 2002, in the fall or early winter since it was mostly dark around 5:30-6pm mountain time.I was walking my bike home from work, did not own a car at the time, it was uphill on a bike path in Boulder, Colorado. It was fairly dark or just after dusk. I like to stargaze, so when I was walking I was looking up every now and then. I have done this since childhood.I noticed a really dark spot against the sky moving from north to south. I know it was N to S because I was facing the mountains, they are to the west. At first I looked away, I don't know why. I didn't really think it was anything I guess. I looked back up and at this point it was almost directly above me. I could definitely make out a triangle shape with very defined edges. There were no lights. There was no sound. No detail could be seen, just a plain very black triangle shape. It moved quickly at a constant rate of speed, but not that much faster than a normal airplane I guess. It seemed like it was only a few thousand feet above me, but I really couldn't determine the distance. It kept going in a straight line and I lost sight as it continued to the South, behind tress, houses, etc. I felt nothing when I saw it or right after the "event". I almost convinced myself I didn't see anything and I didn't tell anyone for a few weeks. It seemed like a non-event to me at first, like it was just supposed to be flying by, like there's nothing unusual about it at all. It was flying over Boulder, a decent size city in Colorado, so I figured it might be military. Also, I was in a dark area on the bike path looking up at the sky. If I was in a normally well-lit area there's almost no way I could've seen it. Under a street light you can't see very far into the night sky and wouldn't have been able to see the stars either, and especially not a black object passing in front of them. That's it. No idea what it was, not convinced it wasn't military since it didn't do anything weird. Who knows.