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After the first clear evidence of the recession—at very high speeds—of the spiral nebulae was announced by V.M. Slipher in 1914, as a result of his work started in 1912, it still took several decades to properly understand the phenomenon in terms of an expansion of the Universe. Some historical issues around that crucial discovery and the contemporary attempts at determining the scale of the visible Universe will be discussed. Presently, very important questions to answer are: What is thedoi:10.3390/universe6110189 fatcat:ooph3gugnnajfkzrj4limveq5u