Like 99% of particle physicists, and 95% of cosmologist (perhaps 98% of theorists and 90% of observers, to be more specific), I was relatively certain that there was precisely enough matter in the universe to make it geometrically flat. What does geometrically flat mean? Well, according to general relativity it means there is a precise balance between the positive kinetic energy associated with the expansion of space, and the negative potential energy associated with the gravitational attraction of matter in the universe so that the total energy is precisely zero. This is not only mathematically attractive, but in fact the only theory we have that explains why the universe looks the way it does today tends to predict a flat universe today.
Now, the only problem with this prediction is that visible matter in the universe only accounts for a few percent of the total amount of matter required to make the universe flat. Happily, however, during the period from 1970 or so to the early 1990's it had become abundantly clear that our galaxy, and indeed all galaxies are dominated by 'dark matter'... material that does not shine, or, as far as we can tell, interact electromagnetically. This material, which we think is made up of a new type of elementary particle, accounts for at least 10 times as much matter as can be accounted for in stars, hot gas etc.. With the inference that dark matter existed in such profusion, it was natural to suspect that there was enough of it to account for a flat universe.
The only problem was that the more our observations of the universe improved, the less evidence there appeared to be that there was enough dark matter to result in a flat universe. Moreover, all other other indicators of cosmology, from the age of the universe, to the data on large scale structure, all began to suggest a flat universe dominated by dark matter was inconsistent with observation. In 1995, this led my colleague Mike Turner and I to suggest that the only way a flat universe could be consistent with observation was if most of the energy, indeed almost 75% of the total energy, was contributed not by matter, but by empty space!
As heretical as our suggestion was, to be fair, I think we were being more provocative than anything, because the one thing that everyone knew was that the energy of empty space had to be precisely zero. The alternative, which would have resulted in something very much like the 'Cosmological Constant' first proposed by Einstein when he incorrectly thought the universe was static and needed some exotic new adjustment to his equations of general relativity so that the attractive force of gravity was balanced by a repulsive force associated with empty space, was just too ugly to imagine.
And then, in 1998 two teams measuring the recession velocity of distant galaxies using observations of exploding stars within them to probe their distance from us at the same time discovered something amazing. The expansion of the universe seemed to be speed up with time, not slowing down, as any sensible universe should be doing! Moreover, if one assumed this acceleration was caused by a new repulsive force throughout empty space that would be caused if the energy of empty space was not precisely zero, then the amount of extra energy needed to produce the observed acceleration was precisely the amount needed to account for a flat universe!
Now here is the really weird thing. Within a year after the observation of an accelerating universe, even though the data was not yet definitive, I and pretty well everyone else in the community who had previously thought there was enough dark matter to result in a flat universe, and who had previously thought the energy of empty space must be precisely zero had completely changed our minds... All of the signals were just too overwhelming to continue to hold on to our previous rosy picture... even if the alternative was so crazy that none of our fundamental theories could yet account for it.
So we are now pretty sure that the dominant energy-stuff in our universe isn't normal matter, and isn't dark matter, but rather is associated with empty space! And what is worse (or better, depending upon your viewpoint) is that our whole picture of the possible future of the universe has changed.. An accelerating universe will carry away almost everything we now see, so that in the far future our galaxy will exist alone in a dark, and seemingly endless void....
And that is what I find so satisfying about science. Not just that I could change my own mind because the evidence of reality forced me to... but that the whole community could throw out a cherished notion, and so quickly! That is what makes science different than religion, and that is what makes it worth continuing to ask questions about the universe ... because it never fails to surprise us.