While flying in a USN EA-18G @ 20k, we saw a large bright white glowing orb flying toward us, getting brighter on a collision course

My pilot and I had just finished training in the Boardman Target Complex MOA in Northern Oregon (near the Dalles). At 7pm Local time, we departed the airspace to the Northwest at 20,000' MSL, headed for NAS Whidbey Island. At approximately 705pm, my pilot asked me what the bright light at 10 o'clock was. I looked out and saw a large white glowing orb at what looked to be maybe 5-7 miles. As we studied it, it started moving on what looked like a collision course with us, and at co-altitude. It got about twice as bright during this time. We then asked ATC for a climb to 24k because we were worried it would collide with us. We received permission to climb and described the position to ATC as we were looking at it, but ATC said they had nothing visible at all on radar. After ATC said that, the craft seemed to drop quickly below the undercast cloud layer at 18k below us. We did not see it again after that.