The first missions don't appear, because they only exist to teach the player, how to play(At least, it was what I supposed).
And yes, dark matter is fairly easy. Most of my levels were actually solved faster in DM, than in normal.
Either way, the campaign is pretty fast to play, a few cookies and some milk on the table, makes it possible in 3/4 sittings ^^.
And you always have costume levels ;-)