Daytime black wing-like configuration flying low and at rapid speed over DenverIt was a mostly sunny, breezy, warm day. My teenaged daughter and I were in our car, just about a block away from my house, when from the top part of my windshield I saw a very rapid West to East (right to left) flyover of something I could not identify. I ducked but they had passed over some trees and I yelled to my daughter to look to the left, as the "object" came through a clearing. Whatever it was, it had seemingly transformed from two closely-spaced but possibly separate black Vs (almost like birds flying closely together) into one slightly tilted black line with 4 distinct, shallow Vs connected to a straight black line making up what appeared to be a wing configuration (no fuselage seen, i.e. it appeared to be a horizontal line while flying away from us). It was flying at tremendous speed -- much faster than any small aircraft would. During the lapse of 1 or 2 seconds between my first seeing it out the windshield to seeing in a different shape (different perspective?) from my driver's side window, it was by then probably about a half mile away and flying at no more than a mile in altitude, yet still about a full inch from side to side at arm's length. Therefore, it was good-sized, yet did not appear to be any kind of typical aircraft -- and not two very fast-flying birds which was what my split-second initial view of it had reminded me of. My daughter was also amazed and said, "Mom, follow it! Turn up here and let's chase it!" By the time we got to the end of the block, I turned, but with buildings and trees obscuring our view, we never caught sight of it again. I rolled down my window, expecting to hear some kind of sound, but didn't hear anything. I would say it was definitely unidentified, low flying, and large, with a totally unique shape, and my daughter thinks it was a military aircraft.