1. INTRODUCTION

I am curious to know why some discoveries, which we now see as very important, made relatively little impact at the time when they were made. This issue is related (but not one to one) to why some people don't get the credit for their discoveries. I will start this talk with two major discoveries related to dark matter in galaxies which got little response from the community at the time

• Zwicky and the mass of the Coma cluster (1933, 1937)
• Kahn & Woltjer (1959) on the timing mass for the M31-Galaxy system

and then contrast these discoveries with the later discoveries of pulsars (1967) and dark matter in dwarf spheroidal galaxies (1983), both of which had an immediate impact. Then I will turn to Slipher's work on galaxy redshifts.