How to take advantage of under-development of opposing pieces? Gambit openings are usually based on sacrificing pieces to have much better piece development. 

Some gambits, like Queen's gambit, or King's gambit, I can understand the idea. However, I cannot evaluate Danish gambit. White sacrifices two pawns, has two developed bishops, but Black seems to be able to cover up pretty fast.

[fen ""]
1. e4 e5 2. d4 exd4 3. c3 dxc3 4. Bc4 cxb2 5. Bxb2


Although it is obvious that Black has no piece development, how can White play to take advantage of it?
This is an example for how it can go wrong for Black side.

[Event "?"]
[ECO "B02"]
[White "Jacques Mieses"]
[Event "Monte Carlo"]
[Black "Frank James Marshall"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/ b - - 0 1"]

[Setup "1"]

1.e4 e5 2.d4 exd4 3.c3 dxc3 4.Bc4 cxb2 5.Bxb2 d6 6.Ne2 Nc6
7.O-O Be6 8.Bd5 Nf6 9.Qb3 Qc8 10.Nf4 Nd8 11.Bxf6 gxf6 12.Nh5
c6 13.Re1 Be7 14.Qf3 Rg8 15.Nxf6+ Bxf6 16.Qxf6 cxd5 17.exd5
Rg6 18.Qh8+ Kd7 19.Nc3 Bxd5 20.Qe8+ Kc7 21.Nxd5+ Kb8 22.Rac1
Nc6 23.Rxc6 1-0