I was by myself at a little after 2:00AM and had just turned west off of I-29 on to Highway 44. I was on my way home after being with friends in Sioux Falls earlier that whole evening. After a couple of minutes of driving west on this little 2-lane highway at about 70 mph, I noticed a light to my left over a plowed field that appeared to be about a hundred feet away. I looked back out the windshield in front of me and realized it was a totally clear night with stars in the sky, and no clouds or moon--in other words it was pretty dark other than this light to my left not too far(Guessing 50 to 100 feet) away from the car I was driving. I glanced back left at the light again and realized this was really kind of weird the way in looked. I would compare it to like cigarette smoke hanging rolling around in a round fish bowl and the smoke lighting up and disappearing as it rolls around inside of the sphere. As I was driving west I noticed what ever this was to my left was matching my speed. Thinking what I was seeing had a stupid explanation, I rolled down my window to make sure I wasn't getting a reflection of a light from inside my car that early in the morning that could be causing an illusion. As it turned out I was seeing the same thing with the window open. So I rolled up the window being scared now and stepped on the gas up to see if I could loose what was apparently following me. As I was gaining speed I was watching my speedometer and the rolling light at the same time. After driving about 15 seconds at 120 mph the light just disappeared. I don't think I slowed much below a 100 mph the rest of the way home. Still kind of shook up, after getting home I called the sheriff in Parker South Dakota at 2:20AM,and asked if anyone had reported anything like what I saw... He asked me what I was on which totally pissed me off and I hung up. I have never really talked about this experience again until about the last 10 years. I have also seen many other things that I will continue to list on this site till I have them all down.