How do astronomers measure, calculate, and/or estimate the value of Dark Energy?

[evad99]

All I know is that it involved 'standard candles' (Supernova, I believe), but what process is involved with coming up with a number for Dark Energy?

[StupendousMan]

One way to "come up with a number for Dark Energy" is to measure the large-scale geometry of the universe. There are several ways to do this: one involves measuring the brightness of supernovae at different distances, another the distribution of small fluctuations in the cosmic microwave background. Often astronomers compare the large-scale measurements to the values which would occur if the universe were spatially flat. The differences between "how things would look if the universe were flat" and "how things actually look" can be quantified.

Now, the geometry of the universe depends on the density of matter and energy. We can measure the amount of mass and the amount of visible matter, and use these values to estimate what sort of geometry would result from these two factors alone. We find that the actual geometry is quite different. That difference in geometry can be attributed to "dark energy", and the size of the difference is one way to assign a numerical value to the importance of dark energy on large scales.

As soon as one moves from vague, general terms, like the ones I've used in this post, to more specific ones, one finds a lot of technical jargon and mathematics begins to appear. I suggest that you read one of the many, many, many good popular books on the topic, and then come back and ask more questions.