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Scientists used the Fermi gamma-ray telescope to calculate it over time.
The Fortune Brainstorm: Green with MNN CEO Joel Babbit
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Watch MNN CEO Joel Babbit speak at the Fortune Brainstorm Green.
Welcome PlentyMag.com readers
March 2, 2009, 11:53 a.m.
Content from "Plenty" magazine is available once again.
Goodbye Snail Mail.
December 29, 2008, 11:02 a.m.
Why waste paper and fuel when you can email instead?
Less Paper. More Productivity.
December 29, 2008, 10:57 a.m.
Electronic faxing is faster, easier and saves trees.
EcoSense for Living: Tips to go green and save money
December 11, 2008, 12:59 a.m. by Emily Murphy
Get tips to go green & save money around the house.
13 optical illusions that will blow your mind
5 brilliant mathematicians and their impact on the modern world
Scientists learn how to record your dreams and play them back to you
Why do we see monsters in the mirror?
6 important things that were invented during the Middle Ages
This world map is weird — and weirdly accurate