  Since the popularisation of the Internet in the 1990s, the cyberspace has
kept evolving. We have created various computer-mediated virtual environments
including social networks, video conferencing, virtual 3D worlds (e.g., VR
Chat), augmented reality applications (e.g., Pokemon Go), and Non-Fungible
Token Games (e.g., Upland). Such virtual environments, albeit non-perpetual and
unconnected, have bought us various degrees of digital transformation. The term
`metaverse' has been coined to further facilitate the digital transformation in
every aspect of our physical lives. At the core of the metaverse stands the
vision of an immersive Internet as a gigantic, unified, persistent, and shared
realm. While the metaverse may seem futuristic, catalysed by emerging
technologies such as Extended Reality, 5G, and Artificial Intelligence, the
digital `big bang' of our cyberspace is not far away. This survey paper
presents the first effort to offer a comprehensive framework that examines the
latest metaverse development under the dimensions of state-of-the-art
technologies and metaverse ecosystems, and illustrates the possibility of the
digital `big bang'. First, technologies are the enablers that drive the
transition from the current Internet to the metaverse. We thus examine eight
enabling technologies rigorously - Extended Reality, User Interactivity
(Human-Computer Interaction), Artificial Intelligence, Blockchain, Computer
Vision, IoT and Robotics, Edge and Cloud computing, and Future Mobile Networks.
In terms of applications, the metaverse ecosystem allows human users to live
and play within a self-sustaining, persistent, and shared realm. Therefore, we
discuss six user-centric factors -- Avatar, Content Creation, Virtual Economy,
Social Acceptability, Security and Privacy, and Trust and Accountability.
Finally, we propose a concrete research agenda for the development of the
metaverse.
