In professionally typeset material people use either spaced en dashes – like this – or un-spaced em dashes—like this—but spaced em dashes are unconventional and unprofessional, frowned upon by anyone who knows what they’re doing. Sometimes professionals put hair spaces or thin spaces around em dashes — like this — but on the web those often are rendered improperly, such as on this forum. Try copy/pasting this paragraph into a rich text editor and swapping between a few typefaces to see what the effect should look like.