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The current Lambda CDM model (Standard Cosmology Model) takes into account present Cosmic Microwave Background Radiation (CMBR) measurements which indicate that the universe is flat - not ballon-like in shape. The ballon analogy was invented as an analogue to explain expansion of the universe. Everything expanded all at once including space just after the 'Bang'. An expanding flat rubber sheet is more applicable.
There is no centre. The explosion analogy is not applicable either as explosions do have a centre. They also result in collisions of the exploding particles as well as expansion of the space between others. We only observe expansion (via redshift) at the largest scales of the universe.
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