Matthews is one of the least penalized players in the National Hockey League and players from around the league take liberties on him almost every game with complete amnesty. In this case, Dahlin gets away with two cross checks without being penalized before Matthews chose to retaliate.
The 24-year-old has no prior history of supplemental discipline and the cross check did not result in an injury to Dahlin, so it's likely that if Matthews gets anything at all, it will be a fine or a 1-game suspension. Any more than that will certainly raise some eyebrows, given what the punishments were in similar situations involving Montreal's Jonathan Drouin (5K fine) and Joel Edmundson (not a thing).