General aviation pilot reported seeing an object pass by them in cruise flight; reported the sighting to ATC; and continued the flight.

Narrative

While cruising on autopilot during an IFR flight at FL100; I noticed something shiny approaching fast from the front right of our plane and at the same altitude. The object quickly passed to the right (east) of our position and within 1;000 ft vertically of our right wing. The object's altitude was within 100 ft of ours. My passenger also saw the object.The object was not visible on TCAS/ADS-B and no evasive action was taken. The time from the first sighting to the passing was less than 10 seconds and the object was clearly not going to hit us.The object appeared metallic and shiny. My opinion is that it was a mylar balloon; a weather balloon; or potentially a drone (although it did not appear to have wings and I did not see any propellers). The object was completely shiny and did not have any writing on it. It was too small to be another aircraft; and my guess is that it had a diameter less than five feet.I immediately reported the sighting to Louisville Approach along with my current altitude. Louisville Approach stated that they did not have any targets visible on radar in the vicinity.The remainder of the flight was uneventful. Nothing further to report.