This paper investigates whether supermassive black holes (BHs) in the local universe are relics of active galactic nuclei (AGN) activity. By comparing the local BH mass function (BHMF) with that expected from AGN relics, the study finds that the local BHMF is consistent with the BHMF of AGN relics, suggesting that local BHs were mainly grown during AGN phases. The local BHMF is estimated using correlations between BH mass and host galaxy properties, such as bulge luminosity and stellar velocity dispersion. The density of supermassive BHs in the local universe is found to be $ \rho_{BH} = 4.6_{-1.4}^{+1.9} h_{0,7}^{2} \times 10^{5} \, M_{\odot} \, Mpc^{-3} $. The BHMF of AGN relics is derived from the continuity equation, assuming AGN activity is due to mass accretion onto massive BHs and that merging is not important. The study finds that the BHMF of AGN relics is consistent with the local BHMF, indicating that local BHs were mainly grown during AGN activity. This agreement is achieved while satisfying the constraints imposed by the X-ray background. The comparison between the local and relic BHMFs also suggests that merging is not important in shaping the relic BHMF at low redshifts (z < 3). The study estimates the average radiative efficiency $ \varepsilon $, the ratio between emitted and Eddington luminosity $ \lambda $, and the average lifetime of active BHs. The results suggest that local BHs grew during AGN phases with accretion efficiencies $ \varepsilon = 0.04 - 0.16 $ and emitted at a fraction $ \lambda = 0.1 - 1.7 $ of the Eddington luminosity. The average total lifetime of these active phases ranges from $ \simeq 4.5 \times 10^{8} \, yr $ for $ M_{BH} < 10^{8} M_{\odot} $ to $ \simeq 1.5 \times 10^{8} \, yr $ for $ M_{BH} > 10^{9} M_{\odot} $, but can become as large as $ \sim 10^{9} \, yr $ for the lowest acceptable $ \varepsilon $ and $ \lambda $ values. The study also finds that the BHMF of AGN relics is consistent with the local BHMF, indicating that local BHs were mainly grown during AGN activity. This agreement is achieved while satisfying the constraints imposed by the X-ray background. The comparison between the local and relic BHMFs also suggests that merging is not important in shaping theThis paper investigates whether supermassive black holes (BHs) in the local universe are relics of active galactic nuclei (AGN) activity. By comparing the local BH mass function (BHMF) with that expected from AGN relics, the study finds that the local BHMF is consistent with the BHMF of AGN relics, suggesting that local BHs were mainly grown during AGN phases. The local BHMF is estimated using correlations between BH mass and host galaxy properties, such as bulge luminosity and stellar velocity dispersion. The density of supermassive BHs in the local universe is found to be $ \rho_{BH} = 4.6_{-1.4}^{+1.9} h_{0,7}^{2} \times 10^{5} \, M_{\odot} \, Mpc^{-3} $. The BHMF of AGN relics is derived from the continuity equation, assuming AGN activity is due to mass accretion onto massive BHs and that merging is not important. The study finds that the BHMF of AGN relics is consistent with the local BHMF, indicating that local BHs were mainly grown during AGN activity. This agreement is achieved while satisfying the constraints imposed by the X-ray background. The comparison between the local and relic BHMFs also suggests that merging is not important in shaping the relic BHMF at low redshifts (z < 3). The study estimates the average radiative efficiency $ \varepsilon $, the ratio between emitted and Eddington luminosity $ \lambda $, and the average lifetime of active BHs. The results suggest that local BHs grew during AGN phases with accretion efficiencies $ \varepsilon = 0.04 - 0.16 $ and emitted at a fraction $ \lambda = 0.1 - 1.7 $ of the Eddington luminosity. The average total lifetime of these active phases ranges from $ \simeq 4.5 \times 10^{8} \, yr $ for $ M_{BH} < 10^{8} M_{\odot} $ to $ \simeq 1.5 \times 10^{8} \, yr $ for $ M_{BH} > 10^{9} M_{\odot} $, but can become as large as $ \sim 10^{9} \, yr $ for the lowest acceptable $ \varepsilon $ and $ \lambda $ values. The study also finds that the BHMF of AGN relics is consistent with the local BHMF, indicating that local BHs were mainly grown during AGN activity. This agreement is achieved while satisfying the constraints imposed by the X-ray background. The comparison between the local and relic BHMFs also suggests that merging is not important in shaping the