https://www.gamespot.com/articles/new-pokemon-go-event-features-bonuses-and-fighting/1100-6458648/

The latest live event for Pokemon Go is a test of strength. A special "Battle Showdown" starting today makes Fighting-type Pokemon appear more frequently, gives you some extra XP for battles, and offers other incentives for visiting Gyms.

Brawlers like Machop and Makuhita will show up more in the wild. Through May 14, you'll get a double XP bonus for Gym Battles and Raid Battles. Raid Battles will also grant a guaranteed 3,000 Stardust just for playing, and you'll get at least one Rare Candy for a win. Gyms are also getting various bonuses, like a double Badge multiplier for visiting one that you visit frequently. Spinning a Photo Disc will get you more items as well.