I understand that there are boundary conditions that make open strings behave as if they were attached to D-branes. However, what I do not understand, is the following: What makes such boundary conditions stable? What forces are responsible for that? Are D-branes independent physical objects that may exist even without strings? If yes, are they made of something more fundamental (which clearly cannot be strings themselves), or are they independent fundamental objects just as strings? Are there generally accepted answers to these questions, or is it something that experts still do not really know? Is pure string theory without D-branes inconsistent? Also consider this analogy: D-branes emerge from strings in a similar way as charges emerge from sourceless Maxwell equations. For example, you can consider a central electric field proportional to 1/r^2, which satifsies the sourceless Maxwell equations everywhere, except at r=0. The singular point r=0 can then be interpreted as a pointlike charge. How much sense such an analogy makes?