Seljak completed his secondary education at the Nova Gorica Grammar School and carried out his undergraduate studies at the University of Ljubljana, Slovenia.

[3] Seljak is a cosmologist who is particularly well-known for his research on cosmic microwave background radiation,[4][5] galaxy clustering and weak gravitational lensing,[6] and the implications of these observations for the large scale structure of the universe.

[11] Much of Seljak's recent work has been focused on how to extract fundamental properties of our universe from cosmological observations using analytical methods and numerical simulations.

Notable examples include Fourier-based Gaussian processes for analysis of time and/or spatially ordered data, generative models with explicit physics symmetries (translation, rotation), and sliced iterative transport methods for density estimation and sampling.

Seljak was awarded the 2021 Gruber Prize in Cosmology jointly with Marc Kamionkowski and Matias Zaldarriaga, who together "introduced numerous techniques for the study of the large-scale structure of the universe as well as the properties of its first instant of existence.