The mechanism of dust formation in galaxies at high redshift is still unknown. Asymptotic giant branch (AGB) stars and explosions of supernovae (SNe) are possible dust producers, and non-stellar processes may substantially contribute to dust production, for example grain growth in the interstellar medium (ISM). Our aim is to determine the contribution to dust production of AGB stars and SNe in nine galaxies at z ~ 6-8.3, for which observations of dust have been recently attempted. In order to

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... termine the origin of the observed dust we have determined dust yields per AGB star and SN required to explain the total amounts of dust in these galaxies. We find that AGB stars were not able to produce the amounts of dust observed in the galaxies in our sample. In order to explain these dust masses, SNe would have to have maximum efficiency and not destroy the dust which they formed. Therefore, the observed amounts of dust in the galaxies in the early universe were formed either by efficient supernovae or by a non-stellar mechanism, for instance the grain growth in the interstellar medium.