Shadow pokemon's have recently been introduced in Pokemon Go. the reason for this is that they will return to Sword & Shield.

Shadow Pokemon made an Entrance in Pokemon Colosseum & then in Pokemon XD.

The story is that Shadow Pokemon's are popping up in the Galar Region. The Dynamax phenomenon is invented to defend everyone against the Shadow Pokemon. You will have to find out who invented it yourself in the story. So you will have to deal with Shadow Pokemon several times. Shadow Pokemons can also be found as random encounters this time. In Colosseum and XD that was only possible with trainers. Just like before, the Pokemon will also have Shadow Moves which will be very powerful and some will even injure the Pokemon. Who or what is behind it is a mystery.