According to standard models supernovae produce radioactive ^ { 44 } Ti , which should be visible in gamma-rays following decay to ^ { 44 } Ca for a few centuries . ^ { 44 } Ti production is believed to be the source of cosmic ^ { 44 } Ca , whose abundance is well established .
Yet , gamma-ray telescopes have not seen the expected young remnants of core collapse events .
The ^ { 44 } Ti mean life of \tau \simeq 89 y and the Galactic supernova rate of \simeq 3/100 y imply \simeq several detectable ^ { 44 } Ti gamma-ray sources , but only one is clearly seen , the 340-year-old Cas A SNR .
Furthermore , supernovae which produce much ^ { 44 } Ti are expected to occur primarily in the inner part of the Galaxy , where young massive stars are most abundant .
Because the Galaxy is transparent to gamma-rays , this should be the dominant location of expected gamma-ray sources .
Yet the Cas A SNR as the only one source is located far from the inner Galaxy ( at longitude 112 ^ { \circ } ) .
We evaluate the surprising absence of detectable supernovae from the past three centuries .
We discuss whether our understanding of SN explosions , their ^ { 44 } Ti yields , their spatial distributions , and statistical arguments can be stretched so that this apparent disagreement may be accommodated within reasonable expectations , or if we have to revise some or all of the above aspects to bring expectations in agreement with the observations .
We conclude that either core collapse supernovae have been improbably rare in the Galaxy during the past few centuries , or ^ { 44 } Ti -producing supernovae are atypical supernovae .
We also present a new argument based on ^ { 44 } Ca/ ^ { 40 } Ca ratios in mainstream SiC stardust grains that may cast doubt on massive-He-cap Type I supernovae as the source of most galactic ^ { 44 } Ca .