Over the years, different people had different theories about why the Mona Lisa had no eyebrows. Perhaps it was an artistic choice; perhaps it reflected the fashions of Da Vinci's era. However, in 2007, Parisian engineer Pascal Cotte did detailed digital scans of the painting and found that the Mona Lisa did have eyebrows and eyelashes once; but due to centuries of erosion and restoration efforts they've disappeared. Cotte's research also illuminated other interesting insights about the painting: if you're interested in reading more in-depth, I would recommend this article from the newspaper The Telegraph. If you're looking for some informal "fun facts" about the Mona Lisa, I found this Mental Floss list a fun read. And, of course, if you'd like more general background on the Mona Lisa, pay a visit to the website of the Louvre, the museum in Paris where the painting is displayed.