The Moon has a large number of dark spots, as can be seen on the photo below (Wikipedia), that differ significantly from the lighter areas. The dark areas often seem to be roundish, but not exclusively. What is the source of the darker areas of the moon?

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The dark and large areas are solidified lava beds from earlier periods of Lunar evolution, when it was still volcanically active. Notice that they might be somewhat less covered by still visible impact craters than on average the lighter areas are, as then still liquid lava would completely cover any traces of impacts from the early Lunar history. As the lava eventually solidified with the Moon cooling down, later impacts would of course still remain visible. These large lava beds are called Maria (Latin, pl. for Seas, sing. Mare), for example, Apollo 11 Lunar Module "Eagle" landed in the area called The Sea of Tranquility (Lat. Mare Tranquillitatis), calling the landing site The Tranquility Base.

Smaller dark regions, but still large enough to be clearly observable on the attached to the question photograph and indeed also to the naked eye when observing Luna on the night's sky, are a product of larger meteor impacts that were energetic enough to literally melt the impact area's Lunar surface and penetrate deep enough to cause mixing of the molten rock from the Lunar surface with materials from the Lunar inner, denser layers.

Denser areas of smaller impact craters are however just casting shadows on the Lunar surface, also making them appear darker than the average darkness of the Lunar surface, covered in regolith and with better light reflection properties.

This answer is not based on any external reference. Please, let me know in the comments, if they are required.

Edit to add: Another interesting thing about the darkness of the Lunar surface is that the far side of the Moon has relatively few lunar maria, compared to its near side that faces the Earth at all times due to this Earth's natural satellite being in a tidally locked orbit with our planet. Knowing that, we could argue that the most commonly used term for Lunar far side, the dark side of the Moon, is in fact a misnomer:

    Side-by-side comparison of the near (left) and the far (right) sides of the Moon, taken by the NASA's Lunar Reconnaissance Orbiter.

Additionally, and as luck would have it, NASA has just released a 360 degree view of the Moon, a video of stitched together still images taken by its Lunar Reconnaissance Orbiter, and published for example on this CBS News page.

Thanks go to Paul A. Clayton for pointing this out in the comments below!

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    $\begingroup$ Well it's a misnomer anyway as it gets plenty of light; everyone just knows Pink Floyd way better than tidal locking. $\endgroup$
    – Amory
    Sep 24 '13 at 17:43
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    $\begingroup$ @Amory It should be noted that "dark" can mean things other than unlighted or dark-colored--e.g., referring to lack of knowledge ("I am in the dark about X") or even evil ("dark lord")--the dark side of the Moon is where the forces of evil have located their secret lair :-). $\endgroup$ Sep 24 '13 at 19:26

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