Supernova remnants in our own Galaxy and nearby galaxies are theoretically observable for over a million years. Since the rate of occurrence of supernovae in our own galaxy is now reasonably well determined to be one every 25 years, there should be in the order of 1,000,000 / 25 = 40,000 supernova remnants present; (i.e., if our galaxy has been around for at least a million years?)

1. After much diligent searching, only 205 supernova remnants have been observed!

2. And if you calculate, using the observed rates of expansion and the present radii, how long ago it was that the shell type supernovae explosions occurred, all the dates are under 10,000 years. Whereas, if the universe were really old, one would expect a distribution of ages, ranging from just a few years to over the million years that we calculate that the expected supernova remnants would still be strong enough to be detected via today's radio-telescopes.)

Many surveys of distant galaxies have been made to find the value of t (tau), the average rate of occurrence of supernovae in the various types of galaxies. One of the most striking conclusions of these studies is given by the comments of a team of astronomers writing in a National Research Council hand-book. They stated, in the context of a discussion of the galactic problems of the decade:

Where have all the remnants gone? 1