Counter Matter is a Legendary weapon obtainable from any mob (except Statues) with a 1:25000 drop chance. Destroys nearby enemy projectiles.
After blocking the projectile, your next strike would be 100% critical.
It is a Legendary support item, which means that its stats will grow with every boss killed, including some of the modded ones, for example, bosses from Calamity mod. In this case, Cooldown will decrease.
Please note, that killing same bosses over and over again will not yield you the upgraded stats.