  Experimental and numerical studies of the velocity field of dark solitons in
Bose-Einstein condensates are presented. The formation process after phase
imprinting as well as the propagation of the emerging soliton are investigated
using spatially resolved Bragg-spectroscopy of soliton states in Bose-Einstein
condensates of Rubidium87. A comparison of experimental data to results from
numerical simulations of the Gross-Pitaevskii equation clearly identifies the
flux underlying a dark soliton propagating in a Bose-Einstein condensate. The
results allow further optimization of the phase imprinting method for creating
collective exitations of Bose-Einstein condensates.
