How to make a widget fill remaining space in a Column

If you need just to add blank space, use Spacer()

new Column(
children: <Widget>[
    new Text('Title',
        style: new TextStyle(fontWeight: FontWeight.bold)
    ),
    new Text('Datetime',
        style: new TextStyle(color: Colors.grey)
    ),
    Spacer(),
],

Not 100% sure but I think you mean this. The Expanded widget expands to the space it can use. Althought I think it behaves a bit differently in a Row or Column.

new Column(
children: <Widget>[
    new Text('Title',
        style: new TextStyle(fontWeight: FontWeight.bold)
    ),
    new Expanded(
        child: new Text('Datetime',
             style: new TextStyle(color: Colors.grey)
        ),
    ),
],
)

You can use:

  1. Expanded with Align to position child

    Column(
      children: [
        FlutterLogo(),
        Expanded( // add this
          child: Align(
            alignment: Alignment.bottomCenter,
            child: FlutterLogo(),
          ),
        ),
      ],
    )
    
  2. Spacer

    Column(
      children: [
        FlutterLogo(),
        Spacer(), // add this
        FlutterLogo(),
      ],
    )
    
  3. mainAxisAlignment

    Column(
      mainAxisAlignment: MainAxisAlignment.spaceBetween, // add this
      children: [
        FlutterLogo(colors: Colors.orange),
        FlutterLogo(colors: Colors.blue),
      ],
    )