M-theory is not just populated by strings, but also by membranes called D-branes. These are multi-dimensional surfaces that move through the eleven dimensions of M-theory. We can have D-branes of up to nine spatial dimensions (though that’s a little hard to visualise)! A point is a D0-brane, a string a D1-brane, a sheet a D2-brane and so on.
Eleven-dimensional M-theory can look exactly like ten-dimensional string theory. This happens when one of the eleven dimensions is extremely small and circular. A two-dimensional D-brane wrapped around this extra dimension will look like a cylinder. But if the circular dimension is tiny then this cylinder will be very thin. As a result the D-brane will appear to be a one-dimensional string moving in ten dimensions (see picture).
In recent years D-branes have become increasingly important to research. They are natural places for fixed endpoints of open strings to live. And strings living on D-branes give rise to the same kind of forces that appear in the Standard Model.
But there is an even more potent reason driving interest in D-branes: they are non-perturbative objects. D-branes allow physicists to do calculations that transcend the approximate methods of perturbation theory. Thus we can uncover elements of the theory in regimes where interactions are strong. Historically this was uncharted terrain.
D-branes are a central ingredient in modern research. They can be used to construct cosmological models within string theory. Researchers in brane cosmology build models of inflation based on brane collisions. And the study of D-branes has shed light on some of the most elusive elements in the universe, black holes. Finally, D-branes played an essential role in formulating the AdS/CFT correspondence.