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),
     ],
   )