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dark matter  Nonluminous mass, whose presence can be inferred only because of its gravitational influence on luminous matter. Dark matter may constitute as much as 99 percent of all the mass in the universe. The composition of the dark matter is not known. 
dark nebula  A cloud of interstellar dust that obscures the light of more distant stars and appears as an opaque curtain.
declination Angular distance north or south of the celestial equator. 
degenerate gas  A gas in which the allowable states for the electrons have been filled; it behaves according to different laws from those that apply to "perfect" gases and resists further compression. 
density The ratio of the mass of an object to its volume. 
deuterium A "heavy" form of hydrogen, in which the nucleus of each atom consists of one proton and one neutron. 
differential galactic rotation  The rotation of the Galaxy, not as a solid wheel, but so that parts adjacent to one another do not always stay close together. 
differentiation (geological)  A separation or segregation of different kinds of material in different layers in the interior of a planet. 
disk (of Galaxy)  The central plane or "wheel" of our Galaxy, where most of the luminous mass is concentrated. 
dispersion Separation, from white light, of different wavelengths being refracted by different amounts. 
Doppler effect  Apparent change in wavelength of the radiation from a source due to its relative motion away from or towards the observer. 
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