Finely grate the cucumber, then wrap the shreds in a clean kitchen towel or cheesecloth and wring out as much liquid as possible, until only the pulp remains.
Add the cucumber pulp to a bowl with the yogurt, grated garlic, vinegar, salt, and chopped dill if using. Stir to combine.
Taste and adjust salt and vinegar as needed. Cover and chill for at least 30 minutes before serving. Can be made up to 2 days ahead.