Why does an absorption spectrum (of eg Helium) show dark lines?

Ignoring the dark lines, we have a continuous colour (visible light) spectrum. The dark lines represent where the photon of a particular wavelength is missing, hence we cant see it. This occurs when the light passes through a material, and various frequencies of photons are absorbed. Hence we do not see these specific photons when the light reaches the observer, and therefore we have 'dark lines' on an absorption spectrum.

Notes: 

1) Photons (light 'packets') are absorbed by electrons (in a material)

2) This causes the electron to become excited, moving to a higher available energy level

3) The 'jump' in electron energy levels is discrete, and equal to the energy of the photon; hence only specific frequencies of photon are removed (photon frequency dictates photon energy; E=hf)