1. It's the energy content. The stuff you can see (baryonic matter) is 5%, dark matter is 25%, the rest is dark energy. Technically you should take radiation in

1. It's the energy content. The stuff you can see (baryonic matter) is 5%, dark matter is 25%, the rest is dark energy. Technically you should take radiation into account, but this is negligible in today's universe. The types of energy can be characterized by how it changes as the universe expanda. If it becomes larger by a factor of a, matter density goes down as a^3, radiation density goes down as a^4 and dark energy density stays constant. Energy is not conserved in an expanding universe, so don't worry about that. 2. W is not the cosmological constant, but a related parameter. It does not go anywhere, the density just changes. 3. These things are unrelated. We do look for changes in these constants either way as good as we can, because imagine the excitement! But so far they all look very constant. 4. No one knows. It's probably always been close to -1, or else the universe would look very different, but you can play around with all kinds of time parameterization of w. The math gets complex very quickly, so the most common parameterization is a linearization. Simply measuring that is a challenge, so measuring this quantity in a model independent way is extremely difficult.