Air carrier Captain reported a near midair collision during climb with what they believed to be a UAS.

Narrative

Near mid-air collision with what I thought to be a drone. It passed off the left side of the aircraft between the fuselage and wingtip and low enough to the horizontal plane that the First Office saw it go by. I reported; 'less than 100 feet'; but the reality was maybe 20 feet above and 20 feet to our left and opposite direction. There were no windows or markings. It was a perfectly symmetrical triangle of 3 equal sides of approximately 2 feet each. The corners were each chamfered and the height or thickness was 15 to 18 inches. The entire craft was gloss white with a plastic or fiberglass appearance. There was no empennage or vertical fins. In fact; there were no flat surfaces at all. There was no visible means of propulsion. There is no doubt in my mind this craft was under it's own power and moving every bit as fast as we were. Over the last 50 years of flying I know the difference between balloons; birds; or even debris like a plastic bag I once saw.Cause: I was able to get a really good look at this craft because I was actually trying to pick up an aircraft that was displayed on our TCAS. The target aircraft was 3500 feet above and descending at least 500 feet per minute. We were climbing at a little over 2000 feet per minute and my estimate on the targets distance was approximately 20 miles. I thought it could get close. At first I thought the white triangle craft was my target but the target was still in front of us moving left to right when the triangle craft passed. The controller asked if I thought it was a drone to which I replied; 'I believe it was a drone'. I had no other explanation. I have since performed a cursory investigation into white triangular shaped drones and could not find anything on the Internet that could even come close. Just so there's no confusion; I believe it to be man-made.